This error occurs when your organization has the Advanced Waivers feature enabled in VolunteerHub, but the user's birthday is not a required field in the User Form.

To correct this issue, navigate to User Forms and click on the "All Users" user group. Click the ellipsis (...) button beside the Date of Birth (or Birthday) question and select "Edit Question" from the menu that appears. For the Require a response to this question setting, choose YES and then click Save.