Screen mirroring
Macbook Pro 15 2015 can't screen mirror with iPad Pro 12.9 2018 (Gen 3). Have tried new cables, restarting, reset the trust etc etc...iPad just doesn't show as a screen.
Not sure what to do next?
iPad Pro, iPadOS 15
Sidecar system requirements
Sidecar works with these devices and operating systems.
macOS Catalina or later
MacBook Pro introduced in 2016 or later
Use an iPad as a second display for a Mac - Apple Support
your 2015 model is too old.
that feature requires the H.264 HEVC encoding instructions built into SkyLake and later intel processors.