We weren't able to add this connection--Customize Forms

A user is getting this error (see below) when he tries to submit a list item through a Sharepoint list with a Custom Form. I've reviewed the documentation about sharing a canvas app with users (https://docs.microsoft.com/en-us/powerapps/maker/canvas-apps/share-app) however custom forms don't show up as an app in the Powerapps menu. Not sure what to do. He's listed in all the appropriate permission groups and can access the list itself normally, just can't submit.

Thanks!