You have selected a custom field in both the 'Required Custom Fields' and 'Read only Custom Fields' sections. A 'read only' ...

You have selected a custom field in both the 'Required Custom Fields' and 'Read only Custom Fields' sections. A 'read only' custom field cannot be edited, and so cannot be 'required'. Make sure the custom field is either 'Required' or 'Read only' and try again.