The engine is running and the clutch pedal is "up" and it won't move with the transmission in reverse. It's a gear drive, right? a 124....that must mean the clutch is slipping. Could be the pressure plates are rusted to the drive shaft and are not engaging the friction disk, or the friction disk has disentigrated. In front of the dash column, directly behind the engine, look for the pair of metal plates with a friction disk sandwiched between. Operate the clutch pedal while watching what is happening. The cause might be apparent.
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