The error is
[BUG SWARMS] lua/entities/bug/init.lua:78: attempt to index a nil value
1. unknown - lua/entities/bug/init.lua:78

I'm getting the issue, too. I took a look at the lua and while I can't understand how it works at all it seems that my download is blocking the necessary model info, bugswarm_collisionmodel only appears in the three bug types and nowhere else and there's only one model in the gma, so it must be a faulty or incomplete download.

EDIT: Restarting the game with all other Workshop addons disabled fixed the issue, so it's a conflict with something else. Now to figure out what.

EDIT2: Found the conflicting addon: https://psteamcommunity.yuanyoumao.com/sharedfiles/filedetails/?id=2950719460

I have no clue how these two are even in conflict, it seems like they shouldn't be conflicting at all. It's like a Playermodel conflicting with gm_construct, that's how weird it seems to me...

I don't have tool organizer unfortunately, so I have to do this myself. What makes it worse is that the state of the bugs only changes when restarting the game. If you disable all mods, then bugs will still be errors. If you restart, then enable all mods, bugs will be fine. So I have to restart the whole game every time I want to test.

Found what was causing it. Infinite Pistol Ammo of all things. There seems to be a theme of completely unrelated addons being incompatible with it.

Does that addon have an options thing too? I guess that's what's conflicting.

why do you need infinite pistol ammo 💀