What does it mean for a power supply to have an output range?
It's either two models: one for 12V the other for 24V but they have the same model number, you just have to add 12V or 24V when you order them. Manufacturers are too lazy to write different model numbers.
Or it's an adjustable supply from 12 to 24V. And you can regulate it to 18V. But usualy the adjustable range is narrower than that.
I don't know if it will damage it but the motor will be much stronger, (stronger torque). Maybe too strong. Will certainly overheat and smoke. So it's strongly recommended not to use 24V instead of 18V. 20V may be OK.