Hello @LydiaCarey,
On the item details page for the feature service your submission_url is pointing to take note of the name of the layer your survey should be pointing to.
Back in your XLSForm on the settings worksheet enter the name of the layer from the feature service under the form_id column (B2).
Survey123 by default assumes the name of your survey is the same as the name of the layer your survey is pointing to. This may not always be the case so setting the form_id will force Survey123 to point to the correct layer name.
Thank you,
Zach