Error trying to connect Gmail

I received the following error trying to set up this plugin:

Access blocked: wpmailsmtp.com has not completed the Google verification process.

wpmailsmtp.com has not completed the Google verification process. The app is currently being tested, and can only be accessed by developer-approved testers. If you think you should have access, contact the developer

If you are a developer of wpmailsmtp.com, see error details

Error 403: access_denied

I have used this plugin on a number of website but never have had this particular error.