Charge the battery - or put in a new one.
The battery has voltage enough to make the starter relay but not enough energy available to turn the engine over.
You press the start button - the start relay makes - high current flows to the starter - the battery can't cope and the voltage drops to a very low level - the starter relay drops out because of the low voltage - current stops flowing - the voltages rises enough to set the starter relay and it all repeats.
You hear a click-click clickety click.
It's a weak battery....
Good Luck..., February 2016