Just to let you know, had to start a new save instead of using saves from any of the previous demo's, as couldn't press "n" to start the next waves.
I'm aware, sorry about that! I've written a recommendation on Patron for starting a new save but I forgot to write it here on itch.
The reason is that I've added a lot more content and things that save/load. So the game might try to load something in demo 12 that wasn't present in demo 11, then errors take place.
If you want to keep your gear/stats, the saves are located in "C:\Users\ [YourUsername] \AppData\LocalLow\Niem2D\Femboy Survival" (Replace YourUsername). Create a new Demo 12 save, then you can copy/paste the values you wish to keep into the new file and I think everything should work fine.