Sidecar is a new feature in macOS Catalina that allows an iPad to function a secondary display both wired and wirelessly. It comes in handy if you work with ZBrush, Photoshop, or Adobe Illustrator. It is also nice if you just need some extra screen space or want to take notes on your iPad while doing something else on your Mac. However, it appears that Apple's Sidecar comes with a caveat.
According to an Apple white paper, the feature only works with MacBooks equipped with the notoriously flawed butterfly keyboard. Well, Apple didn't quite put it that way. It said Sidecar is compatible with MacBook and MacBook Pro 2016 or later or MacBook Air 2018 or later. Of course, all of these have the problematic keyboard.
If you have an older model, you are out of luck. The reason is that Sidecar relies on the Intel Skylake chipsets used in the newer model laptops. These chips added support for the HEVC video codec, which is likely what Sidecar uses to push video to the iPad. Apple did not mention that this was the case, but it seems as good a guess as any.
Your iPad must also be relatively new for it to work. All iPad Pros are compatible, but a regular iPad must be sixth-generation or later. Likewise, fifth-gen mini or third-gen Air or newer will also work. All models also require iPadOS 13.
Fortunately, desktop options are a bit broader. If you are running anything later than a 2015 iMac, 2017 iMac Pro, 2018 Mac mini, or the upcoming 2019 Mac Pro, you'll be good to go without the keyboard mess.
So if you were looking forward to using Sidecar, be sure your components fall into the above ranges. Those not wanting to hassle with the funky butterfly keyboard will have to wait until next year when Apple starts phasing them out.