I've met all of the requirements for sidecar, but it is still not working.

I just use the cable that charges the iPad, with a little adapter on the mac side to convert USB-C to USB-A.

It works with Sidecar perfectly, and also in Zoom, allowing me to easily share the iPad screen. Wonderful for teaching remotely (together with the Apple Pencil, of course!)

The adapter on the mac side is similar to the image below, and also allows to connect any other device that uses a traditional USB cable.

Is this your mac: MBP 16"? That is supported, of course.

So: you say you've met all the requirements, but the details do matter... so let's see.

Did you try connecting the iPad pro to your mac directly with a cable?

Are both devices on the same wifi network, both have bluetooth turned on, AND logged in to the same Apple id?