Diagnosis
The disc brake pads are contaminated with oil or debris, or the rotor is glazed.
How to Fix It
Clean the rotor and pads. Remove the wheel. Use isopropyl alcohol and a clean cloth on the brake rotor. Lightly sand the brake pad surfaces with fine grit sandpaper. Clean again with alcohol. Reinstall.
If That Didn't Work
The pads are worn out or the material is of poor quality. Replace the rear brake pads with sintered metal pads designed for electric scooters.
Heads Up
Squealing often means reduced braking power. Test your brakes at low speed in a safe area after any cleaning or pad replacement.