When I try to open the config, it says I need to install FTB Library, but I have it installed. I'm using the newest version 1.16.5 version of JEI.

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I'm having the same issue.

Same issue here too.

glad i'm not the only one. :/

See #2351,
tldr downgrade jei to 7.6.4.90, they merged for FTB that were not in release yet

looking at this issue might help.
#1526
it's not solved but it is a known issue

See #2351,
tldr downgrade jei to 7.6.4.90, they merged for FTB that were not in release yet

So, I tried this, but got an error where Applied requires version 7.7.0.98 or above. Is there another way?