If you're not seeing any active tests when in youtube studio an API key wasn't properly created for the extension, here's some steps you can take to try and create one:
uninstall extension
make sure any ad blockers are disabled
or allow app.thumbnailtest.com and studio.youtube.com
reinstall extension
open app.thumbnailtest.com
wait on thumbnail test for 20 seconds to make sure an API key is generated successfully (there’s no visual indication currently but it’s being worked on)
then open YT studio making sure you’re signed on the correct channel
the overlay should slide in if you have active tests then slide out of screen if everything synced correctly
refresh test page and data should be there