Depending on the Nanoleaf device you have, there are different reasons why you may not see the device on the pairing screen of the Nanoleaf app.

Nanoleaf Essentials – Thread

Bluetooth is used for pairing, so ensure your mobile device has Bluetooth enabled and is within range of the Nanoleaf light. If you're having trouble, try moving your mobile device closer to the Nanoleaf light.
Ensure the light is powered, especially if it's connected to a switch. All Nanoleaf lights turn on when powered out of the box.
If your device is a HomeKit light (non Matter), for security reasons, the pairing window might be closed. Reset the device to put the device back into pairing mode. See reset instructions below.

Nanoleaf Light Panels, Shapes, Lines, Elements, 4D, Skylight, Blocks

Bluetooth and Wi-Fi are used for pairing, so make sure your mobile device is on a Wi-Fi network.
Devices that are on firmware under version 12.1.0 relies on Apple HomeKit for device discovery (for iOS users only). If you have trouble locating your device on the Nanoleaf App, try pairing in the Apple Home app. Note: The pairing window closes after 10 minutes.
For security reasons, the pairing window might be closed. Reset the device to put the device back into pairing mode. See reset instructions below.
If your Nanoleaf light is already on a Wi-Fi network, ensure that your mobile device is on the same network.
If you're on the same network as the Nanoleaf light but still can't see it, your router configuration may be preventing your mobile device from communicating with the Nanoleaf light. Some common fixes for this in router settings include:
1. "AP Isolation" (also known as "Client Isolation" or "Guest Mode") must be disabled. When enabled, devices on the Wi-Fi network can communicate with the internet but not with other devices on the local network.
2. "mDNS" or "Bonjour" is a device discovery protocol used by many apps, including ours, to find your devices. Ensure this is enabled in your router settings. If you can't find it, most routers have this enabled by default.

Bluetooth and Wi-Fi are used for pairing, so make sure your mobile device has Bluetooth enabled and is within range of the Nanoleaf light. If you're having trouble, try moving your mobile device closer to the Nanoleaf light.
If your Nanoleaf light is already on a Wi-Fi network, ensure that your mobile device is on the same network.
Ensure the light is powered, especially if it's connected to a switch. All Nanoleaf lights turn on when powered out of the box.

Note - Click here for firmware update instructions for Nanoleaf Wi-Fi Bulbs on FW 2.2.1.