For some reason, the mod that i made is not working in the server because:
Failure message: VSSMP (vssmp) has failed to load correctly
java.lang.RuntimeException: Cannot find class net/minecraft/client/model/HumanoidModel
And the singleplayer is working normal, can someone pls help me?

Looks like you're classloading client only classes in the server, where they're missing. This is most likely a mistake on your side. You should make sure not to load client only code on the server.

Join a modding discord for more help, as Github is not an ideal place for support