Hi! I have a little problem

Item textures that I used as Internal item type are buggy on version 1.12.2

My server core version is 1.15.2, I use ProtocolSupport to support below versions

Is there some way to fix this error? Would be very grateful!

I will close this until more comments are sent.

Players who join in minecraft 1.12 need the resourcepack for minecraft 1.12, regardless of the version of your server. If players can join from different minecraft versions, you need to send each of them a different resourcepack (in this case, you will need a more complicated mechanic than simply a server resourcepack, since you can only have 1 server resourcepack). If you are lucky, you can simply generate these resourcepacks by Exporting for a different minecraft version. Note: the .cis file needs to be exported for the minecraft verison of the server. Also, KnokkoCore must match the version of the server.

Supporting players from multiple minecraft versions is probably a very difficult task and I would be surprised if anyone has actually managed to do this before. Of course, you are free to try it.