can't download Portal Xbox live gold game, all buys on xbox.com ends with "Sorry Your...

Hello! it may be a temporary issue. Try again later or tomorrow and see if it works.

It's a temporary issue on Microsoft's side, all we can do is be patient and wait

I was gonna say make sure your credit card isn’t expired, but it seems other people are having the same issue. If it doesn’t get resolved in a day or so contact Microsoft & see what they can do.

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I've had this happen before and after trying and trying like you have, I just left the Store and went back to playing a game for a while. I went back to the Store later on, or it may have even been the next day, to download that game and then it worked.
It might just be a case of coming back to it later if none of the things you are trying is working. Sorry and good luck to you.