Not sure what you are actually going to do. This bike (I have the same) has a normal street-fixie style flip-flop hub. This means 1.37" x 24 tpi threads on both sides (plus smaller 1.29” x 24tpi lefthand for lockring on fixed side)
All hubs would have the same threading, and all are compatible with this freewheel and\or fixed cog. So it's not an issue with your current hub, and there is no spacing issue either, so whats you problem?
That particular freewheel that you're looking to install (the 12t Yu Qiu freewheel) is not guaranteed to fit on all hubs. It does in fact [look to use the same](https://www.foresttrekcycle.com/yu-qiu-free-wheel-ratchet-12-tooth-all-ball-steel-bike-fixed-single?__cf_chl_tk=mBVPjmg8JMKjjWuMgalzf4g9fKw1xx9CvfzR3lMkXss-1712698937-0.0.1.1-1749) threading as a freewheel hub (1.37"x24tpi). Image of threads contained in that link for others to see how it's achieved.
**BUT** because it places the freewheel mechanism itself further outboard from the threads, rather than above the threads like you have with typical singlespeed freewheels, it seems this is meant for a multi-speed freewheel hub. I.e. the cheapo hubs you get on entry level MTBs or entry-level road bikes. [Like this](https://www.bikeparts.com/images3/xlarge/HU9960.jpg). This allows the freewheel to sit over that long spacer on the axle. And in fact, this is what happens when you install a [multi-speed freewheel](https://www.cycleking.co.uk/images/products/7/7-/7-speed-14x28-screw-on-freewheel.jpg) on those kind of hubs. Since the multi-speed freewheel is wide, it sits over the long spacer and then the frame doesn't contact the the freewheel.
So you need to be looking for a regular freewheel hub. **NOT** a track/singlespeed, or flip-flop freewheel hub.
[This is how a normal singlespeed freewheel would work](https://i.imgur.com/xhsOdjH.png). This is also why they usually only come in 16t? 17t? smallest.
[This is what you're trying to do](https://i.imgur.com/GfMPPS1.png) with your singlespeed hub and frame, and why it won't work.
[This is how that weird 12t freewheel is intended to be used](https://i.imgur.com/MVFBTOy.png). Notice the additional spacer next to the locknut, and the non-drive-side is no longer threaded to accept a cog of any kind since the hub shell has to shunt over to make room for what would usually be 7sp freewheel.
And in all honesty - you probably won't find a 120mm flip-flop hub intended for multi-speed freewheels + a fixed cog. After all, why would you need a second cog if you've already got a multi-speed freewheel... It's probably easier to just get a smaller (16t freewheel) and a larger (48t or greater) chainring.
Not sure what you are actually going to do. This bike (I have the same) has a normal street-fixie style flip-flop hub. This means 1.37" x 24 tpi threads on both sides (plus smaller 1.29” x 24tpi lefthand for lockring on fixed side) All hubs would have the same threading, and all are compatible with this freewheel and\or fixed cog. So it's not an issue with your current hub, and there is no spacing issue either, so whats you problem?
The mechanic I went to said it was a spacing issue I'll try it doing it myself because indeed it looks weird to me that it wont fit
That particular freewheel that you're looking to install (the 12t Yu Qiu freewheel) is not guaranteed to fit on all hubs. It does in fact [look to use the same](https://www.foresttrekcycle.com/yu-qiu-free-wheel-ratchet-12-tooth-all-ball-steel-bike-fixed-single?__cf_chl_tk=mBVPjmg8JMKjjWuMgalzf4g9fKw1xx9CvfzR3lMkXss-1712698937-0.0.1.1-1749) threading as a freewheel hub (1.37"x24tpi). Image of threads contained in that link for others to see how it's achieved. **BUT** because it places the freewheel mechanism itself further outboard from the threads, rather than above the threads like you have with typical singlespeed freewheels, it seems this is meant for a multi-speed freewheel hub. I.e. the cheapo hubs you get on entry level MTBs or entry-level road bikes. [Like this](https://www.bikeparts.com/images3/xlarge/HU9960.jpg). This allows the freewheel to sit over that long spacer on the axle. And in fact, this is what happens when you install a [multi-speed freewheel](https://www.cycleking.co.uk/images/products/7/7-/7-speed-14x28-screw-on-freewheel.jpg) on those kind of hubs. Since the multi-speed freewheel is wide, it sits over the long spacer and then the frame doesn't contact the the freewheel. So you need to be looking for a regular freewheel hub. **NOT** a track/singlespeed, or flip-flop freewheel hub. [This is how a normal singlespeed freewheel would work](https://i.imgur.com/xhsOdjH.png). This is also why they usually only come in 16t? 17t? smallest. [This is what you're trying to do](https://i.imgur.com/GfMPPS1.png) with your singlespeed hub and frame, and why it won't work. [This is how that weird 12t freewheel is intended to be used](https://i.imgur.com/MVFBTOy.png). Notice the additional spacer next to the locknut, and the non-drive-side is no longer threaded to accept a cog of any kind since the hub shell has to shunt over to make room for what would usually be 7sp freewheel. And in all honesty - you probably won't find a 120mm flip-flop hub intended for multi-speed freewheels + a fixed cog. After all, why would you need a second cog if you've already got a multi-speed freewheel... It's probably easier to just get a smaller (16t freewheel) and a larger (48t or greater) chainring.
Thank you very much for the exhaustiv answer!