The 1-2 shift gate is out of position and is either stuck in first gear or it is part way engaged but enough to lock out the other gates. You need to pry it up so the 1-2 slot lines up with the other two shift gates, reverse and 3-4. Once you get all three centered, manually move each shift gate one at a time with a pry bar or large screw driver to make sure that they move one at a time and stop positively in their shifted position. In other words, they should not just flop back and forth. There is a detent ball and spring to stop the gate in each position, like 1, N, and 2.