Cant dupe with a universal dupe 1.21.11 dupe on a 1.21.4 singleplayer hosted world.

Cant dupe with a universal dupe 1.21.11 dupe on a 1.21.4 singleplayer hosted world. it gives me an error and kicks me but doesnt dupe for some reason. and also i cant install plugins on the singleplayer world. why?