Here is the error code I am getting when trying to publish a form set to a new modern list. There is nothing fancy in the form, just new form set for a new list. Will export and try again somewhere else to see if it's environment wide or just this list.
Unable to save form sets
{"error":{"code":"-2147024891, System.UnauthorizedAccessException","message":"Attempted to perform an unauthorized operation."}} Please check the application console (CTRL+ALT+I). If you need help, contact our support team.