Word Merge blog gives message like: Creating document or attachment failed.Entity: OpportunityID:Mandatory: Opportunity

When merging the message "couldn't create letter" will force you to check the Word Merge Log [Documents / Reports / Word Merge / Log]. If the message is like "Creating document or attachment failed.Entity: OpportunityID:Mandatory: Opportunity" one should realize that the Word Merge Template is linked to a document type. And a document type has links, so in this example the link to the Opportunity had been set to Mandatory, but the Opportunity field didn't even appear or has been filled in the request from which you're merging. Then you'll get this message, so check the document type. 

Keywords

 "Creating document or attachment failed. Entity: Opportunity ID: Mandatory: Opportunity"