Both rear brakes seized Toyota Highlander

[Richard Kirshy]

Suggest you clamp off the soft line(s)
Attempt to loosen the bleeders and try to compress the piston(s).
If there is no movement, you most likely have seized calipers.
If the caliper pistons retract with some effort, you may have a fault further forward:
Faulty brake hoses(s)
Kinked brake hard line
Faulty master cylinder