Welcome to JUST ANSWER small engines, let me try to help you !!
Do you have a voltmeter to test the battery?
Battery should have minimum of 12.5 volts with key off, then with leads still on the battery, turn to to start. Does it drop below 8 volts?
If voltage drops below 8 volts, the battery does not have enough voltage to turn engine.
Does this mower have electric pto for blades?