If CreativeCore Mod is not working, start with the error shown by Forge, Fabric or NeoForge. A missing library, an incompatible version and duplicate files need different fixes. Record the message before changing your installation.
Fix a missing CreativeCore dependency
Open the mods folder of the instance you actually launched. Confirm that the CreativeCore library .jar is directly inside it and has not been extracted or left inside a ZIP archive. A source archive is not the installable library.
Fix an incompatible CreativeCore version
Compare the Minecraft version, mod loader and requested library range with the installed filename. Replace the mismatched file with a compatible build. Fabric, Forge and NeoForge downloads are separate and are not interchangeable.
Remove duplicate CreativeCore files
Keep one intended CreativeCore library in the instance. Move old library versions out of the mods folder before testing the replacement. Check that your launcher is not restoring an older modpack file during startup.
Check a crash after installing CreativeCore
Read the first relevant dependency or exception message in the crash report. Note the mod it names and the expected version. CreativeCore appearing in a log does not by itself establish that the library caused the crash.
Test a smaller Minecraft instance
Back up your modpack, then test a separate instance with the same loader, CreativeCore and the dependent mod, including any other required libraries. If that combination launches, compare it with the larger pack to narrow down the conflict.
Download a matching CreativeCore build
The CreativeCore Mod downloads list the supported Minecraft version and loader for each hosted file. Choose a replacement only when those details match your instance and dependency requirements.