"Unverified payment method" simply means that the client has not yet entered their payment method with Upwork (usually a credit card) and had UW verify it. That status is very common for clients using UW for the first time because it's not worth their time to go through the verification process until they find a freelancer they want to hire.
I never hesitate to submit proposals to clients with unverified payment method, but I never accept a contract until their payment method is verified.
