SideCar was the reason I was excited for Catalina. But alas, the feature is not supported on 2015 Retina Macbook Pros, only the newer ones with the horrible keyboard, pointless touchbar, only one kind of port, etc.
I'm in the same boat. Installed the betas on both iPad and MBP about a month ago. Turns out my 2015 pre-horrible-keyboard MBP is too old. And none of the published hacks around the restriction work nowadays.