Draft ID Column Not Configured
You are trying to start a merge job in Merge to Draft or Draft to Send mode but the Draft ID column is not configured in the output configuration. Draft ID column must be configured for these modes. Merge to Draft mode inserts draft IDs into that column. Draft to Send mode use those draft IDs to send them.
Add a new column for example Draft ID and map it in the output configuration in the Draft IDs section. Then you will be able to start a merge job in Merge to Draft mode and add-on will insert draft IDs into the selected column. After that you can send drafts using the Draft to Send mode. Once a draft is sent add-on removes it's draft ID from sheet.