Two mods ship the same library and collide
Applies to Forge, NeoForge.
What it looks like in your log
java.lang.module.ResolutionException: Modules kotlin.stdlib and kotlinforforge export package kotlin to module
What causes it
Two mods bundle conflicting versions of the same Java module, and the JVM refuses to load both.
How to fix it
Update the two mods named in the error; if the clash persists, remove one of them.
Not sure this is your problem? Paste your log and Roastd will tell you which of these it actually matched, and what else it found.
Check your own log