Was this screenshot taken on a desktop pc or mobile device? Maybe Google pay can be used through the mobile app as payment if it acts like Apple Pay.
Edit: I just tried adding funds on Safari on my mobile device and it seems google pay is working.
You can add funds, but you cant buy directly from the store, at least is my case
I suspect there’s some caching bug issue. Refresh your browser. Select Card option and back to Google Pay then try again.
Still happening after clearing cache, with Chrome and Firefox
Google pay has been asking me to provide document proof of my payment method, so that could be what’s happening here. Go to pay.google.com and see if you have a notification in the top right corner of your screen saying this (pictured below). Other than that, I don’t know what else could be going on. Just thought I’d share what’s going on with me in case it can help you.