I have only one available slot. I tried to troubleshot the hardware and it said detection failure. But I can noit figure out how to make it to detect the card.
If you have only one available because another card is in the other slot, try swapping them.
One of those mysteries I came across years ago is that some cards just didn't like some slots. Way less than an ideal situation, and fodder for Mac fan boys, but it is what it is.