Idk why it won't work is my car missing something
98 nissan maxima se 3L auto trans

 

Check your fuses, some fuses if they blow will cause loss of OBII connection

 

is the key turned to ON?

also to rule out a bad reader, car you read codes off a different vehicle?

 

My 97s obd2 port doesn't work with some readers. I think it's that the port is loose and the connector on some readers don't fit well. My ultragauge won't work unless I shove a piece of paper to snug up the connector.

You can try to wiggle the connector in the port and see if anything changes. If it sometimes connects it may just be the same issue I have on mine.