Seems that some other people are reporting the same problem on the forums. No idea what to do at this point. I can try contacting Support, but I understand the wait times are ridiculous. Bleh. Any suggestions?
I hope it's not the same bug I encountered more than a year ago (and complained in steam thread about). I couldn't buy anything on steam for close to a month, I contacted steam support it did nothing, of course, I got some autoreply that was utterly useless.
On the bright side, after some random update it reverted back to being as it should.
edited: I was getting the same message as you when trying to buy.