Dotnet 3.1.100 under Raspbian 4.19.75 not writing GPIOs

I’m following this article which tries to flash LEDs, but I can’t get it to do that.
I’ve checked that I get 3.3v and 5v out of right pins from the breadboard, and that the LED lights when I connect it to the grounds. I’ve also checked that running the project under sudo doesn’t make a difference.

What else do I check?

Got it. Needed to to an apt-get update and upgrade.

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