This error typically occurs when responses are deleted from the spreadsheet.
Form Assembly created a tab to hold the responses and labeled it something like
You might have tested your form a few times and then deleted those test responses from your spreadsheet. Form assembly doesn’t like that and will complain on the next response with this error message.
Just delete the “empty” tab.
There is usually still a header line, but zero response rows means “empty”
Now the next response will automatically re-create the needed
Revision: ## tab and add the response.