After countless hours of reinstalling mods and trying pre-made packs every time I try to play on my local server, it says "The Server must have JEI installed" when it has the mod already in the mods folder.

Please help

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Thanks for the report!
I suspect something in JEI is crashing, or some other mod is having a bad interaction with JEI.
I need some more information to solve this issue, please paste your full fml-client-latest.log or latest.log to https://gist.github.com and link it here.