Why can't I get my alternator back in my truck?

If you got it out, you should be able to wiggle it back in, if it's the same replacement part. Keep at it. sometimes I walk away for a bit and go back to it. you'll figure it out eventually.

There may be a little sleeve thingy in the bolt bore that needs to be moved , prying may not do it easily

Compare the old alternator to the new one. You might have a unit that was boxed wrong. It happens. Hope that helps! Jim