Sidecar says the device is locked or asleep

Why it happens at exactly the wrong moment

On a Mac that uses the iPad as its only screen, the Mac finishes starting long before anyone touches the iPad. The first connection attempt therefore lands on a sleeping device and comes back with this error, which internally is error −203.

Retrying immediately does not help; the iPad is still asleep. Retrying forever with a dialog each time is worse, and that is what naive automation does.

How to stop seeing it

1. On the iPad, Settings → Display & Brightness → Auto-Lock → Never, while it serves as a display.
2. Keep the iPad on a USB cable so it never runs down.
3. Let SideMini handle the waiting. It retries in short bursts, then watches quietly, so waking the iPad connects it within a few seconds without a single dialog.

If it never connects even when awake

Then the error is masking something else. Work through the iPad is not in Screen Mirroring, or read the fuller Sidecar troubleshooting guide.