If you have selected the checkbox field "Require Membership No." and the membership number field is not being validated, then please check:

• Payment Item Object, has validation rule "Check_for_Valid_Membership_No" enabled
• On the Payment Form, the "Event Type" field is NOT blank or "Ticket Only"
• Note, the validation occurs on the specify attended page only and not on the first screen that has the tickets selection.  This is because the membership validation is at the Individual contact level and all attendees are required to have a valid membership no.

For more details on this, please see Turn on the "Require Membership No." validation rule on the Payment Option guide.