The following prerequisites are required, unfortunately you cannot use any iPad or Mac:
Macs that can utilize Sidecar in Catalina:
iPads running iPadOS 13 that can act as a second monitor using Sidecar:
Any of the current iPad range including:
Both the Mac and the iPad will need to be logged into the same iCloud account.
This can also be done from System Preferences, click Sidecar.
(If you cannot see the Sidecar option in System Preferences then your Mac doesn’t support Sidecar)
To mirror the Mac’s display, go back to the AirPlay menu on the Mac, which is now a blue rectangle when you are using Sidecar. Select the option to mirror the Mac’s display.
Your iPad is now set up and working as an additional screen providing valuable extra screen space when you are working on your current project. Or alternatively use screen mirroring to share your Mac’s screen with another person.