If you are logged in with multiple accounts under the same browser session, you need to log out from all your accounts before using YAMM.
Open your Google Sheets. Click on your profile picture on the top right side.
If you are logged in with multiple accounts in the same web browser, you can see other accounts below your main account. Click Sign out of all accounts.
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 click on it to sign in again.
You can now start your mail merge as usual!