I haven't tried buying Marriott points, but attempts to buy Hilton and Starwood points (all 3 go through points.com) recently have been met with the same message.

Data points:
- attempts were made from Europe from legitimate IP
- attempts were all within the past, and over a span of, 5 months
- attempts to purchase were from 3 different SPG and 1 HH accounts, all of them in good standing, opened for years, and nowhere near the yearly purchase limit
- tried US-issued Amex and Visa, European Mastercard

My last successful points.com purchase, using the same IP and credit cards as above, was in February 2018. Even then, I was getting this same rejection message on most of my purchase attempts, this since late 2017. Never a problem before that for many years.

My take on this is that points.com, for whatever reason (probably fraudulent purchases using stolen credit card numbers), quietly started blocking purchases from non-US IP's, while pretending the problem is with the credit card, which it certainly is not. They are also using fairly advanced methods to detect use of VPNs with US IP.
I very much resent their approach as the card approval goes through, and the amount remains blocked for a couple of weeks.
They should just clearly announce that points purchase are only available for specified markets.