Not every Mac that supports macOS Catalina will support the Sidecar feature. I could not find any official info, but here are my own findings. There’s an explicit check that the Mac is newer than these models:
- MacBook, MacBook Air, MacBook Pro: Early 2015
- iMac: Late 2015
- Mac mini: Late 2014
- Mac Pro: Late 2013
I managed to make Sidecar work with my Mid 2014 MacBook Pro work and iPad Pro. It’s great in terms of minimal lag, but the image quality is bad. I guess the limitation is about hardware HEVC encoding which requires Intel Core 6th gen processor.
Here’s what it looks like:
https://imgur.com/gallery/wIyv6Xl
That requires disabling System Integrity Protection and editing SidecarCore private framework to exclude your Mac model from the blacklist.
If you need this feature on an older Mac despite low image quality I suggest filing a bug report with Apple. A hidden option to disable such hardware check would be great.