This error results when the ID designated as master object in the Conga Composer button URL is incorrect.
This could result from the ID number (e.g. RU-0001) being selected as the master object ID by mistake when Conga needs the record ID (e.g. a00G0000007MAXg) in order to function correctly.
WHAT TO DO
Ensure that the field you have selected in the button URL is the record ID for the record you’ve put your button on. If you’re using a Salesforce standard object as your master object ID, the record ID field will be listed as Account ID, Opportunity ID, Contact ID, etc. If you’ve put your button on a custom object, the record ID field will be listed as Record ID.
This will be listed as Record ID if you’re placing your button on a custom object.
The &id=parameter designates which object will be your master object.