my car won't go into gear. I've changed the transmission on 1990 Oldsmobile Cutlass Ciera

tried to put it in gear to go to the store and it wouldn't. What could be the problem?

your park reverse and nuetral safety switch might have gone bad check your shift indicator when you put the car in drive manually shift it into 1st gear then as you change speed shift it 2nd and then third. fyi each one of those gears in that transmission work with the torque converter and the pcm and if the pcm is not reading correctly it will send a error to the transmission causing it not to shift properly.