TL;DR: if you do have the right .NET Core (or .NET I guess) runtime installed, install any patch updates or reinstall the latest version if there aren’t any.
Detail:
Similar to a couple of other answers where they just didn’t have the right .NET core installed. I was trying to run a .NET Core 3.1 web app which had worked fine previously for months, and this suddenly started happening.
I did have 3.1 (runtimes 3.1.21 and 3.1.22) installed. However a new one (3.1.23) had been released 12 days earlier, and installing that fixed the problem.
I have no idea if this was because it’s aware that there’s a new patch and I didn’t have it so it wouldn’t run, or if there was just something wrong with my 3.1.22 installation. Worth trying installing latest patch, or reinstalling existing installations.