"Type Mismatch" Driving me crazy

So, the "Type Mismatch" error pops up every time I press "Enter". I have no idea what's causing it. There are no consequences when I ignore it. I press "Enter" I get "Type Mismatch", I press "Okay", and then go about my business until I press "Enter again".

Explain like I'm five.

I'm not sure where the error is coming from, there are 12 modules. Is there no way to search for what's causing it? My BEST guess as to where it might be is here:

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I'm not sure where the error is coming from, there are 12 modules. Is there no way to search for what's causing it? My BEST guess as to where it might be is here:

Please Login or Register  to view this content.

I'm not sure where the error is coming from, there are 12 modules. Is there no way to search for what's causing it? My BEST guess as to where it might be is here:

Please Login or Register  to view this content.