I think you need a TRS adaptor, as the output from the amp will be mono not stereo ie Tip (+) and ring (-) Both those cables you have pictured are stereo cables, something like tip (left channel) ring (right Channel) ring (earth/return)
Although your headphones may be stereo I'm pretty certain the output from the amp wont be.
https://smile.amazon.co.uk/C2G-3-5mm-Stereo-Female-Adapter-Silver/dp/B002DWAXKU/ref=sr_1_19?keywords=stereo+mono+adapter+3.5mm&qid=1577384784&sr=8-19
Something like this in your amp, the plug one of your pictured cables into that, and other stereo end into headphones, should sort it!