Server-side mod won't work

Hi, I was making a server-side mod for one of my friend's servers. I wanted it server-side only so that you can join without any mods. So in MCreator in the workspace settings I checked the server mod option. I also followed your article on how to make server-side mods, and it seems to be all right.

When I run it on your server in MCreator, it works fine. But when I put it on my Scalacube server and try to connect, it immediately disconnects. Sometimes it also says that the mods are incompatible or something like that. But without the mod loaded, it works fine.

The mod was for 1.17.1, and on my Scalacube server, I installed Forge for 1.17.1 (version 37.0.105).