Mailmeteor might have issues authenticating your account. Most of the time, it happens when you have multiple accounts logged in Google.
Here are the kind of error messages related to this issue:
Drive.google.com refused to connect
Authorization failed / connection error
We're sorry something went wrong
Add-on is not showing up in the menu
Not able to install
Still having an issue and need a helping hand? Contact us. We'll be happy to help!