Crank / Balancer spacer
The sleeve should slide into place easily. If you have recently had the front cover off the engine that might be the problem, the front cover bolts need to be tightened with the sleeve installed to align the front seal. If you don't do this a severe oil leak results.
Otherwise you might have rust on the inside of the spacer, wrong size key in the crankshaft, rust or paint on the crankshaft or some sort of damage to one of the parts. Like I said before, the sleeve slides on easily, no force should be used or needed.
Royce Peterson