Do not substitute the current hosted file
The direct download on this site is CreativeCore 2.14.16 for Fabric and declares Minecraft 26.3 or newer. That is not an appropriate match for a Minecraft 1.16.5 Forge profile.
Version numbers and loader names are part of compatibility, not optional labels.
Check the dependent mod requirement
Start with the mod that requires CreativeCore. Its release notes or dependency message should specify the compatible CreativeCore range and loader family for Minecraft 1.16.5.
Use that information to choose the corresponding CreativeCore build from an appropriate archive or source.
Keep older profiles isolated
Use a separate launcher instance for 1.16.5 so modern libraries do not get copied into the same mods folder by mistake.
An isolated profile also keeps configuration files and Java settings easier to troubleshoot.
Read startup errors in order
If Forge reports a missing or incompatible dependency, solve that first error before changing unrelated mods. One incompatible library can cause many later failures.
Write down the exact mod IDs and version constraints shown by the loader so you can compare them with the files installed.
Backup before changing a legacy pack
Older modpacks may depend on a tightly matched set of versions. Back up the entire instance before replacing a core library.
After the change, launch a copy of the pack and verify saves, menus, and dependent mods before continuing regular play.
Current download on this site
The direct file currently hosted here is CreativeCore 2.14.16 for Fabric. Its included metadata requires Minecraft 26.3 or newer, Fabric Loader 0.19.5 or newer, and Fabric API.
Download CreativeCore Mod