Please follow the below steps when you experience issues recording voiceover on a MAC computer:
Close Toonly if it is still up and running.
Go to System Preferences > Privacy & Security.
On the Privacy Tab, select Microphone.
You should be able to see Toonly on the list of apps. Please make sure the box is ticked beside it.
Click Accessibility to see if Toonly is on the list. If not, add it manually by clicking the + sign.
Restart the computer and open Toonly again.