Apple TV Remite (2nd Gen)

If you have a phone (or other bluetooth device) in your pocket/on your person, then put that some distance away. It may interfere with detecting and identifying the intended remote device.

Charge the remote for a bit so that it has enough energy to function normally during the pairing procedure.

From: How do I pair a new Siri Remote with my existing Apple TV?

1. Update your Apple TV to the latest tvOS. Even with auto-update on, it may not automatically have the latest.

2. Position the Siri Remote so it’s within 3 to 4 inches (8 to 10 cm) and pointing at the front of the Apple TV 4K/HD.

3. Press and hold the Back button (“<“) and Volume Up button ("+") buttons for 3 seconds.

4. When the Siri Remote is successfully paired, an onscreen message appears.

When you pair a remote, any other previously paired remote is automatically unpaired.