Replace starter...still does not engage motor

Sounds like the solenoid has gone on your starter motor, this is the device that throws the gear out of the starter motor to engage the flywheel to crank over the engine. Remove starter motor and bench test to confirm, how to do this is: starter motor removed and in vice, earth the body of motor and where the thickest input wire was is main 12v add a 12v to this, and the small input wire just tap 12v to it and see your motor work if not working then replace starter motor, it should throw gear out and spin

Sticking "Dog" the part of the starter motor that engages with the stater ring. Remove Starter motor, check the spring on the dog ( the shaft that comes out of the starter motor) its a faff to repair best replace starter motor

Check power to starter motor solenoid, solenoid operation and starter motor voltage during cranking. If the starter motor is receiving power from the solenoid, but isn't cranking, then replace the starter motor.

Also check the condition of battery terminals, wiring and connections. Also check the earth wire from the body to the engine.

You will need a new starter rebuilt ect. to replace the one you currently have that is defective. you sound like you are spinning the starter motor but the starter bendix is no longer engaging the flexplate/flywheel on the engine.

The solenoid is what makes the starter gear pop out to engage the flywheel to turn the motor. If the solenoid is bad, the starter gear won't pop out. If the starter gear does pop out, then the solenoid is good. The starter motor should turn even if the solenoid is bad.

The one way clutch that engages the starter motor to the flywheel is chattered or damaged and needs to be replaced. The starter motor turns a small gear that is connected to a large gear by a small chain. On the side of the big gear is a smooth machined surface that the one way clutch grabs when the starter is engaged. The surface is chattered and the three grabber rollers wont jam or grab on the surface unless it is smooth therefore the starter spins but the motor does not. You will probably have to replace the large gear and possibly the three rollers also.

If the starter motor remained engaged when you released the key, the underlying cause is likely a bad starter solenoid (first item the positive battery cable connects to after the battery) Remaining engaged, can damage the starter itself. Replace the solenoid, check the starter and you should be finished. (Or also replace the starter if it was damaged)
If the starter was stuck and not engaging, the bendix drive inside the starter has failed...In that case it's best to change the starter as even though it's replaceable, generally by the time it needs replacement the starter is worn in other areas and would need complete overhaul.

solenoid on starter is sticking and needs to be replaced or repaired

Possible solution. You probably know that harleys, unlike almost all the others used a bendix type to shove the starter motor's gears into engagement with the starter clutch. Since you replace the solenoid, the thing that pushes the gears into engagement; the battery, that would push the starter motor if the gears were in engaagement; and the starter motor, which would turn the ring gear if it was in engagement. So wht's left? Unfortunately it sound like it might be the teeth on either the ring gear or the bendix gear, probably the ring gear as it was notoriously weak on iron sportys. Not a problem you want to try to fix at home unless blessed with tools, room and time.

Bad switch or Solenoid. depending on the bike.