Why won't my truck go into gear?

What is your question?

My truck starts and runs just fine. When I push in the brake pedal, the gear shift button will not go in and I can't put it into a gear. I have a shifter in between the two front seats. Lately if I try a couple times, pushing in the brake, the button will finally go and it will go into gear. But this morning it wouldn't go at all. Is this something with the brake or the transmission?