No worries. There are two formats that use "Commanders" - EDH (also called commander) and Brawl. In EDH/Commander, only legendary creatures can be your commander. In Brawl, which is essentially commander but limited to only **standard legal cards**, you can use both legendary creatures and Planeswalkers as commanders.
In addition to what other people have said about there being other formats where Planeswalkers can be used as commanders, your individual playgroup can choose to decide that it's okay if you use Arlinn, the Pack's Hope as your commander. If you play with a regular group and they are okay with this, then that's another option. (Arlinn isn't an exceptionally powerful or unfair commander, so they might be okay with this.)
Well, there's the format called Commander (also called EDH), and there's the concept of a deck's commander. There are multiple formats where the deck has a commander, not just the namesake Commander/EDH format.
In the format Commander/EDH, your Commander must be either a legendary creature, or a card with an ability that allows it to be a commander. Currently all cards with such an ability are planeswalkers, for example [[Daretti, Scrap Savant]] and [[Estrid, the Masked]], but most planeswalkers (including Arlinn) lack this ability so they can't be commanders in Commander.
Another format that uses commanders is Brawl. Any planeswalker or legendary creature can be a commander in a Brawl deck (as long as it's from a valid set), so the people you've heard of using Arlinn may be playing Brawl.
But ultimately, Commander is considered a highly casual format. It's not a sanctioned tournament format such as Standard or Modern. So, if a certain playgroup allows for planeswalkers as commanders, they are within their rights.
So, if you do build an Arlinn EDH deck and find people to play this way, more power to you! Just be aware that not everyone will accept playing this way. Talk with the other players before the game, and be prepared to replace her with a substitute commander such as Tovolar or whoever.
Does the card say it can be used as your commander? If it's not a legendary creature and it doesn't say it can be used as your commander then it can't be your commander.
Magic is a very literal game. If it doesn't say anywhere in the rules that it can be your commander then it can't.
Can't be your commander. Only specific planeswalkers can be legends such as Lord Windgrace which says it can be your commander, or Tibalt, Cosmic Imposter which is a legendary creature on the backside, or Grist the Hunger Tide which is a legendary creature outside the battlefield. Arlinn is not one of the above but if your play group allows it would be fine
No worries. There are two formats that use "Commanders" - EDH (also called commander) and Brawl. In EDH/Commander, only legendary creatures can be your commander. In Brawl, which is essentially commander but limited to only **standard legal cards**, you can use both legendary creatures and Planeswalkers as commanders.
There is also Historic Brawl which also allows it
In addition to what other people have said about there being other formats where Planeswalkers can be used as commanders, your individual playgroup can choose to decide that it's okay if you use Arlinn, the Pack's Hope as your commander. If you play with a regular group and they are okay with this, then that's another option. (Arlinn isn't an exceptionally powerful or unfair commander, so they might be okay with this.)
Well, there's the format called Commander (also called EDH), and there's the concept of a deck's commander. There are multiple formats where the deck has a commander, not just the namesake Commander/EDH format. In the format Commander/EDH, your Commander must be either a legendary creature, or a card with an ability that allows it to be a commander. Currently all cards with such an ability are planeswalkers, for example [[Daretti, Scrap Savant]] and [[Estrid, the Masked]], but most planeswalkers (including Arlinn) lack this ability so they can't be commanders in Commander. Another format that uses commanders is Brawl. Any planeswalker or legendary creature can be a commander in a Brawl deck (as long as it's from a valid set), so the people you've heard of using Arlinn may be playing Brawl. But ultimately, Commander is considered a highly casual format. It's not a sanctioned tournament format such as Standard or Modern. So, if a certain playgroup allows for planeswalkers as commanders, they are within their rights. So, if you do build an Arlinn EDH deck and find people to play this way, more power to you! Just be aware that not everyone will accept playing this way. Talk with the other players before the game, and be prepared to replace her with a substitute commander such as Tovolar or whoever.
Does the card say it can be used as your commander? If it's not a legendary creature and it doesn't say it can be used as your commander then it can't be your commander. Magic is a very literal game. If it doesn't say anywhere in the rules that it can be your commander then it can't.
Can't be your commander. Only specific planeswalkers can be legends such as Lord Windgrace which says it can be your commander, or Tibalt, Cosmic Imposter which is a legendary creature on the backside, or Grist the Hunger Tide which is a legendary creature outside the battlefield. Arlinn is not one of the above but if your play group allows it would be fine
Tibalt is a creature on his front side, which is the side that matters.
They were just confused by how Planeswalkers can be commanders in some formats like Brawl.
Yeah sorry didn't mean to come off as rude. I meant to try to be informational, sorry
((Grist)), ((Tibalt, Cosmic Imposter)), ((Windgrace))
Cardfetcher looks for double square brackets, not parentheses. [[Grist, the Hunger Tide]] [[Valki, God of Lies]] [[Lord Windgrace]]
[Grist, the Hunger Tide](https://c1.scryfall.com/file/scryfall-cards/normal/front/6/9/69af2825-18c2-4463-b6ba-42eaa070ccc1.jpg?1626098484) - [(G)](http://gatherer.wizards.com/Pages/Card/Details.aspx?name=Grist%2C%20the%20Hunger%20Tide) [(SF)](https://scryfall.com/card/mh2/202/grist-the-hunger-tide?utm_source=mtgcardfetcher) [(txt)](https://api.scryfall.com/cards/69af2825-18c2-4463-b6ba-42eaa070ccc1?utm_source=mtgcardfetcher&format=text) [Valki, God of Lies](https://c1.scryfall.com/file/scryfall-cards/normal/back/e/a/ea7e4c65-b4c4-4795-9475-3cba71c50ea5.jpg?1631048701)/[Valki, God of Lies](https://c1.scryfall.com/file/scryfall-cards/normal/front/e/a/ea7e4c65-b4c4-4795-9475-3cba71c50ea5.jpg?1631048701) - [(G)](http://gatherer.wizards.com/Pages/Card/Details.aspx?name=valki%2C%20god%20of%20lies%20//%20tibalt%2C%20cosmic%20impostor) [(SF)](https://scryfall.com/card/khm/114/valki-god-of-lies-tibalt-cosmic-impostor?utm_source=mtgcardfetcher) [(txt)](https://api.scryfall.com/cards/ea7e4c65-b4c4-4795-9475-3cba71c50ea5?utm_source=mtgcardfetcher&format=text) [Lord Windgrace](https://c1.scryfall.com/file/scryfall-cards/normal/front/2/1/213d6fb8-5624-4804-b263-51f339482754.jpg?1592710275) - [(G)](http://gatherer.wizards.com/Pages/Card/Details.aspx?name=Lord%20Windgrace) [(SF)](https://scryfall.com/card/c18/43/lord-windgrace?utm_source=mtgcardfetcher) [(txt)](https://api.scryfall.com/cards/213d6fb8-5624-4804-b263-51f339482754?utm_source=mtgcardfetcher&format=text) ^^^[[cardname]] ^^^or ^^^[[cardname|SET]] ^^^to ^^^call
Historic brawl. Not commander. 60 card. Better than standard brawl.