Diagnosis
The scooter's Bluetooth module is in a deep sleep or has glitched. It's a common firmware quirk, especially after the scooter has been off for a while.
How to Fix It
Force the Bluetooth module to wake up. Turn the scooter's display ON, then hold the power button for 10+ seconds until the display turns OFF. Immediately press the power button again to turn it back on. Now try pairing in the app.
If That Didn't Work
Delete the scooter from your phone's Bluetooth device list, force-close the Apollo app, and restart your phone. Then repeat the scooter power cycle and try pairing again.
Heads Up
The Bluetooth connection is only for app functionality (trip data, settings). It is not needed for the scooter to operate. Do not leave Bluetooth scanning on during rides as it can drain your phone battery.