As FBJ and Pop-Kat says:
Earth fault or Starter motor fault,
If you've got a multi-meter check for volts at the starter solenoid terminals you'll probably need three hands. put the black lead of the meter to the battery earth. Then the red to one of the starter solenoid terminals . The one that connects to the battery will have 12volts permanently.
Swap to the other solenoid terminal. Keyswitch on, Kill switch on, then thumb the starter button. you should see 12volts.
If you do see the 12v at each terminal then the fault could well be with the starter motor brushes, or the earthing of the starter motor.
If there are no volts on the terminal to the starter motor, then disconnect the wire from the starter button, it should be Yellow with a Green trace colour. as before thumb the starter button, again you should see 12 volts on the feed from the handlebar switch. If you do not then there is a fault in the feed to the switches.
The earth fault may well be between the electrical mounting plate and the frame/battery negative terminal, or the solenoid and the mounting plate.
Personally that would be my first place to look. with the multi meter set to Ohms or Resistance, switch off the ignition, (best to disconnect battery as well) find the wire that connects the starter button to the solenoid and disconnect. Then measure the resistance from the end of the wire on the solenoid to the electrical mounting plate, then to the frame ground, or the battery negative lead. Anything above a few ohms is going to give you grief
If you are feeling brave you could also use a suitable metal tool to short circuit the two starter solenoid terminals. That puts the battery volts direct to the starter motor, but be careful not to short it to the frame, as that tends to get a bit exciting* with sparks n stuff!
*Sorry that's an old electrical joke
