DIAGNOSIS The label maker's Bluetooth module is either already paired to another device or needs a fresh start. It can only maintain one active pairing at a time.
FIX You need to clear its pairing memory. Turn the label maker on. Go into your phone's Bluetooth settings (not the Phomemo app), and look for any existing "Phomemo" or "D30" connection. If you see one, tap the info icon and select "Forget This Device." Now, turn the label maker off. Turn it back on, and immediately open the Phomemo app and start the pairing process from there. Don't try to pair from your phone's system menu—let the app handle it.
IF THAT DIDN'T WORK Restart both devices completely. Power your phone off and on. For the D30, perform a factory reset: turn it on, go into its system settings menu (using the buttons), find "Reset All," and confirm. This wipes all pairings and custom templates, so it's a clean slate.
HEADS UP The D30's Bluetooth range is very short, about 10 feet with a clear line of sight. Keep your phone within arm's reach during the initial pairing and during use for a stable connection.