Ubiquiti UniFi U6 Pro access point will not connect one device

The checks in more detail

A device that sees the network but cannot join may be refused by one of the network's settings. Open Settings, WiFi, choose the network and check the security: WPA3 can exclude older devices, so for a test use WPA2 or the WPA2 and WPA3 transitional mode. Check band steering and minimum signal limits. Features that push devices to 5 GHz or disconnect weak clients can strand devices that are far from the access point or that only support 2.4 GHz.

Smart plugs and cameras often need 2.4 GHz, so ensure the network offers it; the Optimize for IoT or a dedicated 2.4 GHz SSID setting can help. Check the minimum RSSI and the beacon settings if you have turned them on. Make sure the network is not hidden or limited by MAC filtering, and that the device is not blocked in the client list. The access point has a limit on the number of clients, so a crowded access point may refuse new ones. Look at the DHCP service.

A device that joins but gets no address, shown as 169.254, means the gateway's DHCP is not reaching it, so check the VLAN assigned to the network and the switch port settings. A network on a VLAN that the switch does not pass gives exactly this. Check the channel width: a very wide channel in a crowded area can cause drops. Update the access point's firmware through the controller. Test with another device to learn whether the issue is the client. Forget the network on the client and rejoin. If the access point becomes unadopted again and again, replace the cable and check the PoE budget. Ubiquiti support or the community forum can help with logs.