I can't connect my iMac 2011 (high Sierra) to my MacBook Pro mjlt2 2015 (Monterey )

You can not. Sorry.

It used to be possible, but is not possible with newer Macs. Remember it is a computer, not a monitor. Good monitors are now available at reasonable costs.

Ronasara is likely correct...

Use your iMac as a display with target display mode - Apple Support

You may be right, I don't know. But one thing I have learned here on this forum is that it is very unwise to go against things that Apple says. They may occasionally make mistakes, but they then rectify them. Going off and doing "unusual" things often creates other, sometimes serious, problems. I suggest you keep that in mind. As I said, monitors are not that expensive.

Yes, it's just that I've seen a couple of posts where the OP said they did it outside of what Apple says is possible. :)

Good advice. :)