That is just a warning not an error. You should still be able to process the payment via Google Pay. I believe you can get rid of the warning by doing the following:
From a Home screen, swipe up or down from the center of the display to access the apps screen.
Navigate: Settings. Apps. .
Tap. Menu icon. (upper-right).
Tap. Special access. .
Tap. Install unknown apps. .
Select the unknown app then tap the. Allow from this source switch. to turn on or off .