We just upgraded to 10.2.600.9.
In the Payment entry, creating a new group, selecting as account “Mastercard”
In the Payment method, the dropdown does not allow any choices to choose from.
Bank Account setup and Payment Method are both found in AP / Setup or in AR / Setup
The Payment method may have scope of AP, or AR.
A Bank Account can have AR and AP payment methods set up.
In Bank Account Maintenance, you add one or the other from the File menu.
Once you add a new one, the selections for AR will be any of the Payment methods you set up as Scope AR
and selections for AP will be any of the Payment methods you have set up as Scope AP.
One other thing, I noticed your screen capture of AP Payment entry. The choices you get on that Payment Method dropdown will depend on what AP payment methods you have set up for Bank Account called Mastercard.