If the following 3 conditions applies:
You are using a Mac.
You are using Chrome.
You are unable to record (either web app or extension) and are either getting an error that "screen recording was cancelled" or no message at all.
Please follow these steps to resolve:
On your Mac go to Settings → Security & privacy → Screen Sharing.
Check off the Google Chrome box and then check it back on.
Relaunch Chrome (it will ask to as well).
Recording should now work.