Well, the constraint clearly says that patient_num being NULL is legal. What is not legal is to give a value to patient_num if payment_card_seq_num is less or equal to 67940.
I think this is constraint you need:
CHECK (NOT (patient_num IS NULL AND [payment_card_seq_num] > 67940))
I have often found it easier to write CHECK constraints by explicitly expressing what I am disallowing.