Will not start

I had bought (used) the same mower, and had the same problem.

What it turned out to be was a safety switch (that won't allow fire to the plug unless either the catch bag or the mulch plug is installed), wasn't working!
BUT...mine wouldn't get fire even if the bag/plug was installed!

On the same side as the carb is on, there should be a wire bundle (running underneath the carb) covered by a corrugated plastic wire covering, AND a single wire with a connector right about even with the carb.
This single wire is for the safety switch.
CAREFULLY unplug the connector, separating the single wire, and try starting the mower.

If it starts (or you NOW have fire to the spark plug), the problem is the switch.

While the switch could be bad, it might also be dirty.
When I removed the plastic cover over the switch, the area was covered with dried out grass clippings.
I used a 3" paint brush and a can of compressed air (like one uses to clean a computer), and cleaned it all out.
When I reconnected the switch wire, it worked as it should.

If the switch IS bad, you can just leave the wire disconnected and run it that way, BUT the safety feature (bag/plug must be in place) won't work.