Hello, so version 1.16.5 I can't use the configuration button. It keeps saying 'Install the Configured' (what I can't find) mod to access the in-game config. jei-1.16.5-7.7.1.137.jar is installed

Configured.

Very first hit for "configured mod" on DuckDuckGo, Bing, and Google.

Thanks for the report!
You can also go to Configured by clicking the text in chat.
Sorry it's confusing, I wish Forge's built-in version of this was working.