Error: The DELETE Statement conflicted with the REFERENCE contraint "FK_MergeTemplateGroup_MergeTemplate" document used in letter pack

This occurs when you are trying to delete a document template that exists in a letter pack.

You are trying to delete a document template that is referred to in a that exists in a letter pack.

Locate the letter pack and remote the document template from the letter pack and then you may delete the document without error.