When I try to start the server, this crash appears and I have to remove the mod to proceed.

https://pastebin.com/TArrkmCp

but if you wanna play with mappy on a server you need it on server side, too
If I only install it in my client it says this:

You can ignore that message. You can join without it having Mappy on the server.

Mappy is client only, and should never be added to the server.