Click on Display Settings there, then on the + dropdown, also make sure the iPad and Mac are logged in to the same Apple ID, connected to the same Wi-Fi network, are of a supported model and are running a supported operating system.
Sidecar works with these devices and operating systems:
- iPadOS 13 or later
- iPad Pro (all models)
- iPad (6th generation) or later
- iPad mini (5th generation) or later
- iPad Air (3rd generation) or later
- macOS Catalina or later
- MacBook Pro introduced in 2016 or later
- MacBook introduced in 2016 or later
- MacBook Air introduced in 2018 or later
- iMac introduced in 2017 or later, or iMac (Retina 5K, 27-inch, Late 2015)
- iMac Pro
- Mac mini introduced in 2018 or later
- Mac Pro introduced in 2019
- Mac Studio introduced in 2022 or later
System requirements for Continuity features on Apple devices - Sidecar - Apple Support