Hi and welcome back 🙂
That seems like some strange behavior, so I’d be happy to investigate with you. I checked our logged bug reports but didn’t see any bug relating to the issue you’re experiencing. Additionally, I’m able to edit my Application Key field in Events > Add-Ons APIS > Eventbrite on my local test environment.
I see you checked “Yes” to reverting back to the default WP theme and deactivating all other plugins other than our as recommended in our Testing for conflicts guide, but can you please confirm this?
Also, send over your System Information as a private reply if possible.
Thank you again for reaching out and have a great upcoming weekend 🙂