If you have only 1 single jack, you need to use a splitter cable if you want to use the rear audio ports of the motherboard to split the input which is on the pink port, and the output which is on the green port.
If your computer case has a combo jack on it, make sure that the combo jack cable is connected properly to the motherboard's audio port. If the issue will persist, kindly test the headset with a splitter cable so you can plug it into the rear port of the system. This will isolate if the issue is the combo jack port of the case or something else.