When you want to launch YAMM from your spreadsheet, you get an error message saying that Google Drive refused to connect. This article explains how to solve this issue.
This issue is not directly related to the YAMM add-on but to a Google outage.
First, make sure you are not logged in with multiple accounts. Learn more: How to log out from multiple accounts?
If you still have the issue, log out from your Google account. Then log in again and start to send a mail merge.
To learn more about how to troubleshoot this issue, you can also read Google documentation.