I have 39 credits and no longer work in the US. (Gave up resident card after 10 years in the US). Retired in Norway. Is there any way I could buy the last credit point I need?
You cannot buy your way into Social Security benefits. The only way to earn credits is by working in the U.S. and earning pay where Social Security tax is withheld.
If you only need one credit, this could be obtained by working in the U.S. until you’ve earned $1320.
You cannot buy credits, but work outside the US can count. Norway is one of the countries that work qualifies: https://www.ssa.gov/planners/retire/international.html
This answer extends Ben Miller's answer, and is a universal answer for any country that doesn't have a deal like StrongBad discusses.
You may be familiar with the Social Security part of FICA - an employee pays 6.2% on the first $130,500 and the employer matches.
With a small business (assuming you haven't placed yourself on the payroll as an employee), you pay both sides as self-employment tax.
This is mandatory. Of course it's only mandatory for small business income in the United States. The IRS doesn't really have a way to check that your income really happened, but we can't tell you on this forum to break the law*, as discussed over here on law.se.
So the official word must be: the income must be genuine. You'd be best to look at remote work/telecommuting/ "digital nomad" type work, and check yours and US tax laws to determine which activities would actually be taxable in the US. Understandably, IRS is a little bit grabby, which works in your favor.
So this is how you can "buy in" to Social Security.
* Hum de dum, if you really see a need to obfuscate your income, there's an amusing answer for that. Nothing to see here, move along.