It may contain abandoned mods that are not compatible with the current version.

A common solution is to remove all mods from your computer, make sure they boot successfully in vanilla, and then
Subscribe to the mods one by one again to find the mod that is causing the problem.
procedure:
1. Unsubscribe all.
Otherwise, the Steam client and Prison Architect will automatically download the mod, which wastes manual deletion.
2.Open the mod file folder in "c: \ Users \ [Username] \ AppData \ Local \ Introversion \ Prison Architect \ mods".
Prison Architect itself automatically places the mod files in this folder. There are no files in a typical Steam folder.
3. Delete all the files in the mods folder.
4. Make sure the game works fine without mods.
If you delete the mod file and it still doesn't work, another problem is hidden.
If it works properly, proceed to the next.
5. Subscribe to the mods one by one again and see if the game starts.
If the same issue occurs somewhere, just in case, delete all the mods again and then subscribe to the suspicious mod first.
If you have the same problem, it's probably the cause.
If it doesn't reappear, it may be a compatibility issue with one of the mods.