I had a case which customer was getting this issue creating a form from SharePoint List:
"Access denied. You do not have permission to perform this action or access this resource."
This was fixed granting access to everyone on the list, but checking internally, seems like the process was taking this service account: "email@example.com" And obviously that account does not have permissions. We needed to grant access to that user and then in power apps we refreshed the connection. It was not taking the user account. I had the same problem with a power app that is taking a service account, instead of user account for a SharePoint list as well. Please take a look at it for future releases.