I myself have no trouble with putting a flat on but it is timed to make the steering wheel straight ahead and the rack centered end to end. You don't locate it just anywhere.

Flat not being there screams possibly wrong part, I would be double checking EVERYTHING else before I butchered the part to not be able to return it. Look close at the spline all the way around, some look round but one small tooth is missing to do the same as the flat.......... .........and not obvious until you look for it.