I don't think your doing anything wrong. I have to press that frame lock button to put it back together as well and last time it was a royal pain in the posterior.
I'd call the barrel and receiver one thing myself, and your trying to put it back on the frame. I think that would be the proper terminology but I didn't write that book either.
You should be able to start it on a bit, then you'll have to depress that button with a thin screwdriver, then it will slide most of the way on and you'll have to hit the button again to lock it in.
Best of luck to you.
In Memory of Paul "Dietrich" Stines.
Dad: Say something nice to your cousin Shirley
Dietrich: For a fat girl you sure don't sweat much.
Cue sound of Head slap.
RIP Muggsy & TMan
"If you are a warrior legally authorized to carry a weapon and you step outside without that weapon, then you become a sheep, pretending that JOCKO will not come today."