If a parent selects On behalf of: Myself when composing a message, the message will fail. This is due to the lack of patient ID for the parent in OP. They must choose the patient for whom the message applies.
The practice can turn off the option to Allow Self Registered Person to Send Message to avoid having the parent listed in the On behalf of drop-down menu. This option is located in Portal Management > Patient Interface > Allow Ability to Send Message > Allow Self Registered Person to Send Message.