My first guess is that that the freewheel mechanism is at fault.

You could try yanking the axle and spraying into the freewheel mechanism with a penetrating lube if all you need is a very short term fix…this would melt the old sticky grease while likely setting the ratcheting unit up for serious wear and tear later.

Just get a whole new rear wheel unless the drive train is all worn out. In that case it is way easier to just get another bike.