The left-hand mandrel on the 917.273050 model garden tractor has two belts going to it. One belt takes the drive from the engine, and a second one below it delivers the drive to the other blades of the deck. Both belts are always under tension and either would normally create resistance to turning by hand so be sure to check turning with both belts removed.
It is possible that the deck was assembled with the two pulleys on wrong, which could cause a bind on the mandrel shaft. There is also a spacer that sits between the lower pulley and the mandrel housing that needs to be in place. The spacer is thin and just looks like a washer, but without it the mandrel will bind and resist turning.
Another possibility is that shipping or other damage twisted the deck slightly and the left-hand blade is actually hitting the outer edge of the housing, clearance is small and it can happen. Or, possibly, the blade was installed incorrectly or the wrong blade was installed altogether.