Why does the message Accessory Not Supported continuously display?

Ok, that would be a 1st generation iPad Pro, not a 3rd generation that I stated before (which I was just going on the time you got it.) However, you still didn't verify which keyboard you are using with it. I provided you a link to compare the keyboard models. I stated that I assumed that it is the Smart Keyboard Folio. Again, is that correct?

You mentioned that the Genius Bar was no help. So, they checked out both of these and it worked for them? ... or, it didn't, and they didn't know what was wrong or couldn't offer any solutions?

Ok, if you purchased this iPad Pro new from Apple in 2019, it would be a 3rd generation model.

Would you please look at the following link and identify which exact keyboard you have. Unless you tell me otherwise, I will assume that it is the Smart Keyboard Folio. Is that correct?

Because the accessory that you are trying to use is not designed to work with this device.

What we don't know, as you didn't provide any details, is: what accessory is this, and what are you trying to use it with?

Sorry, but we can only go on what you provided us, which frankly, wasn't much.

Ok, so you have an iPad Pro. Don't know when you actually purchased it, but I will assume that it was recently so that you have the latest model ... which has an Apple Silicon M2 processor. Is that correct? I will also assume that it is currently running the very latest version of iPadOS ... correct?

Which Apple Pencil did you get with this iPad? It would have to be either an Apple Pencil (USB-C) or Apple Pencil (2nd generation). Which is it?

Was this always an issue, or just started to happen recently. For both the Pencil and the Portfolio?

Finally, what have you tried, so far yourself, to resolve these issue so that we don't waste your time with potential solutions that did not already work for you?