There should be some resistance from the valve springs when turning the crankshaft with the sparkplugs. More from compression with the sparkplugs installed.
If gas leaks thru the carb into the cylinder, it will cause a hydrolock and the starter will jam. You would see gas squirting out the emprty sparkplug holes tho.
Push rod being out of place or valves not being adjusted properly would cause a engine lock too.
Are any of the safety switches bypassed? This sounds like when trying to start when the blade is engaged. Make sure nothing is jamming the belts under the engine.
With the sparkplug removed, is the starter able to turn the crankshaft?
Search Youtube for voltage drop test instructions. You will need a multimeter. Basically, you test voltage drop at every connection point in the wiring. Very simple test. You might be getting some volts to the starter but not 12 volts. Possible cause is faulty soldenoid.