It is my guess that the brake tension is released successfully, but the parking brake pad maybe rusted & stuck to the rotor.

You can take the wheel out, caliper out, and beat the heck out of the rotor with a mallet. It may get the rust off.

if that doesn't work, jack up the rear 2 wheels and place the car on the jack stand. Take the wheels out, take calipers out, and rotor guide bolts out. shoot the whole rotor with penetraint (shoot lot of it into guide bolt hole!) Then crank the engine and hit the gas. The idea is to lose everything as much as possible and over power the rust with the engine.

good luck