Mailmeteor is a Google Sheets add-on and sometimes encounters some issue with Google. Most of the time, the reason why the add-on can't be opened is because you have multiple Google accounts logged in at the same time. If that's your case, have a look to this more detailed article.
If Gmail prompts this error after you've run a mail merge, it means that you've reached your sending capacity or that your email was detected as spam. Mailmeteor sends emails using your own Gmail account, this means that when your account reaches this limit, you can't send emails for a period of around 24 hours. Read this guide to learn more about how to fix this issue.
The most frequent issues have been listed here, but you might have found a new bug that we aren't aware of yet. Please contact us and we will do our best to help you.