The iPad is not in Screen Mirroring

Check in this order

1. Same Apple Account, two-factor on. Look at the top of System Settings on the Mac and Settings on the iPad. Different accounts is the most common cause of an empty list.
2. Bluetooth and Wi‑Fi on both. Sidecar uses Bluetooth for discovery even when the picture travels over a cable.
3. Unlock the iPad once. A device that has not been unlocked since boot does not advertise itself.
4. Turn off Personal Hotspot and Internet Sharing. Either one blocks Sidecar.
5. Check the hardware is supported. An iPad that does not support Apple Pencil does not support Sidecar.

If it appears but will not connect

That is a different failure with its own page: the iPad is locked or asleep. If the Mac has no monitor attached at all, see the headless case, where Sidecar connects and then drops a few seconds later.

What SideMini does about it

It never asks macOS to connect to a device that is not currently listed, which is what produces the “Unable to Connect” dialog people see in loops. It waits, watches, and connects the moment the iPad is reachable. The Doctor pane in Settings → Advanced checks the account, Bluetooth, Wi‑Fi and device visibility in one click.