It's hard to imagine for a lot of people, buying games directly on Steam Deck has been broken this long but Valve are finally looking to fix it.
The issue is that certain methods of payment, and probably only in certain countries, require a special pop-up browser window to do some security checks. It was broken at release of the Steam Deck, with an update attempting to fix it back in March, which only fixed it for some.
In a fresh Steam Deck Beta Client Update, the single note is that it "Fixed Store payment methods which require popup windows for processing not working".
Nice to see but that sure took a lot longer than expected for something so essential. It was frustrating, as every time I forgot it was broken, I then had to wait to buy it on my desktop because it was stuck since it didn't fully go through.