Full Error Message: “Only aggregate expressions use field aliasing (rsdoc)”

Reasons you may see this error message:

1. You may be trying to merge a rich text field in that document. If so, then this format is not supported by our app: https://kb.a5documents.com/faq/does-rs-documents-support-rich-field-text/
2. May have an issue with formatting of one of your merge fields in the google doc template or embedded symbol in a table.

Some things to help you test if issue is one of the fields/tables:
-Replace a new google doc ID into that document action (can even be blank) just to make sure the error is not coming from the document action settings. 
-Decode one section of merge fields at a time in the google doc template used. So for example if you had merge field {!Account.Name} you can decode it be removing the first bracket so it would change to !Account.Name}. Then do this one by one or section by section to isolate where the error is coming from. Then you can add the first bracket { back in once you have resolved which field was causing it. 
3. If not one of the reasons above, there may be an issue with some hidden formatting in that google doc template. You can create a brand new google doc and merge fields in manually to recreate your template. Then test and make sure no error. If no error then yoiu know it was hidden formatting and you can finish building your template using the new google doc ID.