Welcome to GitHub @Flamant16000 .
Your issue would be of greater help if you could give more details.

What Minecraft Version, which Forge Version, which core version, would be nice to know, and what exact error message? comes up if you try to start with core loaded?

also have a look at the other issues, and determine, whether yours may the same as an already existing one.
In that case, vote the existing one up or give more details there and close this one.

Thanks!