Hi Teoh,
Replacing demo users with “System.user” may seem to fix the errors, But there is no email field in “System.user” entity and you cannot edit the “System.user” entity as the system module is “read only”. Its always a best approach to use the “Administration.Account” entity for creating users instead on “System.user”. You can add attributes that you want in the account entity or edit it.
Try validating email while saving the account in a microflow like the below one.
microflow validation reference : https://docs.mendix.com/refguide/setting-up-data-validation/#custom-validation-save-button