iPad Pro sidecar not showing

Hi carolfromtimonium,

Thank you for using Apple Support Communities. We understand that you are not able to select your iPad Pro for SideCar with your Mac. We'd like to assist.

As mentioned in the resource linked to your post, Use an iPad as a second display for a Mac, High Sierra is not compatible with SideCar.

"Sidecar system requirements

Sidecar requires a compatible Mac using macOS Catalina or later and a compatible iPad using iPadOS 13 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
•   
• iPad Pro: all models
• iPad (6th generation) or newer
• iPad mini (5th generation) or newer
• iPad Air (3rd generation) or newer"

If you have upgraded to at least macOS Catalina, and are still seeing the issue, make sure you meet the rest of the requirements from the resource.

We hope this helps.

Cheers!