How do I add payment method?

I am trying to add payment method to a form but it is grayed out?

Are you referring to the Girl Get Out Vendor Registration Form? You already have the PayPal Integration in it. That is the reason why the other payment methods are grayed out. You cannot have two or more payment gateways in a form. You have to delete Paypal if you'd like to use another payment channel. The product items are saved for you and should be available once you add the new payment tool.