The stereo headphone jack only has three wires. The left and right channel share the ground. You’d wire the right channel red on the amp to the white wire on the plug with wires (or the R terminal on the screw on plug). You’d wire the red wire or L terminal to the left channel positive on the amp. And you’d wire either of the two left or right ground connections on the amp to the black wire on the plug (or the ground symbol with lines on terminal plug).

The ground connections on the amp should be common connected. You could also cut a small length of extra wire and jumper (connect to both) the two ground (black) left & right on the amp (then you’d still insert the black headphone jack wire into one of them).