Why won't my iPad show up in the devices for Sidecar?
why wont my ipad show up in the devices for sidecar
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Your iPad, if running at least iPadOS 13, is compatible with Sidecar Use your iPad as a second display for your Mac with Sidecar - Apple Support, but you posted in the Mac OS High Sierra area, so, if you're using this OS, it won't work. You need Catalina for it.
In the link I provided you can see all the system requirements for Sidecar to work.