Sometimes it can happen that Yet Another Mail Merge doesn't work as expected. If there isn't any specific solution to your issue, best would be to reinstall the add-on. This article explains how to reinstall YAMM on your Google account.
Open a new or an existing Google Sheet.
Click Add-ons > Manage add-ons.
A pop-up opens, click the three dots button on YAMM icon > Uninstall.
Once done, type Yet Another Mail Merge in the search bar to find the add-on. Click Install to reinstall YAMM.
Learn more on the following article: 'Install Yet Another Mail Merge on your individual account'