Chain brake is on- locking the clutch bell?
Incorrect assembly, something binding?
Something fallen in and lodged between case and inner face of the flywheel?
Strip it back down each side of the crank- flywheel back off, clutch and oil pump back off- check crank rotation then.
If you went as far as having the cylinder off the saw- something could be inside the crankcase jamming crank lobes to case.