problem in my machine lathe TMC250

Hello @ham86 below a quick translation for those of our non English speaking members.

Quick translation :
Notre ami a un tour CNC Huron TMC 250 acheté en 2014.
Quand il appuie le bouton de changement d'outil, la tourelle émet un bruit comme si elle allait changer l'outil mais ne tourne pas.

Hello
Breaking between the stepper motor and the gear of the turret?
Have a good day

I do not know specifically this machine but I think you could look into these possible causes :
- chips or stuff blocking or hampering the turret movement
- loose linkage between stepping motor and tool holder
- weak power supply in the stepper motor circuitry
- damaged wiring or plug
- defective or disturbed position sensor if any

Did it begin suddenly ?
Have you been able to see if motor is moving at all when pressing button ?