Aparently, my newest PC, which has 16 GB RAM, an i7-7740X 8-core processor running at 4.3 GHz, and a NVIDIA GeForce GTX 1080 display adapter, doesn't qualify to run Windows 11.
It has all the necessary security thingies and boot capabilities, but Microsoft have arbitrarily excluded any i7 processors older than 8th generation (mine is 7th gen. - the first number after the i7- or i5- or whatever tells you the generation).
This is not for any performance or lack of capability reasons - it's just that Microsoft would prefer us to buy new PCs, because they make money that way.
Unless Microsoft changes its policy, it looks like I'll be using only out-of-date unsupported versions of Windows after the updates for Windows 10 stop in 2025. I use Linux for most everything now anyhow.