Mailmeteor is a Google Sheets add-on and sometimes encounters some issue with Google. If you can't open the add-on, try to:
Close your browser and re-open the add-on
Open an incognito window in your browser
Try with another browser
One of the top reason where the add-on can't open is where you have multiple Google accounts. 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.