Quick solution
Log out of all Google accounts
Log back in using only the account you need
Try the add-on again
Understanding the error
These errors typically happen when you're signed into multiple Google accounts at once. The most common errors you might see are:
Error 400: This means there's an issue with how your account is connecting to the add-on
PERMISSION_DENIED: This appears when the add-on can't access your spreadsheet properly
Alternative fixes
If logging out doesn't work, try these options:
Share the spreadsheet with your main Google account and switch to that account
Use an incognito window in Chrome and log in with just one account
Make sure you're using the account that originally installed the add-on