If you are logged in with multiple accounts under the same browser session, you will need to log out from all your accounts before using YAMM.
Open your Google Sheets. Click on your profile picture on the top right hand side:
If you are logged in with multiple accounts in the same web browser, you will see other accounts below your main account. Click ‘Sign out’:
To remove all the accounts except the one you want to link to this unique Chrome session, click ‘Remove an account’ > ‘Yes, remove’:
If you want to use YAMM with this unique remaining account, simply sign in again to this account:
You can now start your mail merge as usual!