What version of the game are you using? Did you use a save from an older version?
Well I went to try downloading it again, but for some reason the only files are for v19.1.7, a save file and an evs file.
Also just as an update. I tried taking the 20.2 version I'd downloaded the other day and just put the 20.1 save file in to there, the nuke still didn't work.
Ok, update again. Not sure why Itch wasn't showing the 20.2 version but it came back. I got that and put my load in and the nuke worked that time. Weirdly I loaded right in to a level with the confirmation withoutt having to select it.