Similar to this post, except switching to mobile data did not fix it.
In my specific case, my son was trying to buy Robux. I was able to redeem his gift card using mobile data (as in this post) but when attempting the purchase all I got was "Your Play balance can only be used in Canada".
To be fair, I am using a VPN - with a Canadian ISP address. Turning it off did not help even though I live in Canada and my DNS was correctly reported as being in Canada. Every setting I could find said the account was in Canada.
I was able to fix the issue by switching to mobile data, downloading the Roblox app on my own phone, signing in to Roblox as him, then paying for the Robux using my own payment information.
I could theoretically have done the same thing via a web browser, except that Roblox did not allow purchase of the # of Robux he wanted via web. (On mobile, you can buy 80 Robux; on web, there is no option for 80.)
This solution obviously does not fix the issue of him having a "stranded" Google Play balance that he cannot redeem. I contacted Google about this issue and they had no answer.
I get that Google is trying to crack down on scammers but this is utterly absurd. I certainly won't be purchasing any more Google Play cards, and I highly recommend that no one else does either.