If you receive an unhandled error when creating a letter from a template, we suggest checking on any View type merge fields in the template itself. These merge fields use Views to pull a table into the template, but if the View is using older (or legacy) data or if the View pulls no results, this can generate an error in the system. The easiest resolution of this problem is to remove the View type merge fields from the template to bypass the error and allow the letter to generate.