I was heading to Hays Kansas last night and an American Airlines flight was issued DIR Memphis. They came back with-
We’ll need a vector, we don’t have GPS tonight. WTF? No FMS? no GPS? how is this possible?
An old MD80/A 
Probably. There have been plenty of MD83s that flew into and out of KLAS that were /I… so it’s more than probable that they didn’t have FMS or GPS capability.
BL.
American Airlines? 
Yep. I remember distinctly listening to an AAL MD80 asking for clearance at KLAS, and they mentioned that they were /I, and refused an RNAV clearance.
BL.
It can happen sometimes, the GNSS part of the FMS can be defered ( broken but the plane can still fly with VORs) or the FMS if the planes are single FMS is simply U/S.
Jay
Better whip out your sectional chart and start some serious pilotage! 
Maybe there were some actual pilots on board, not just button pushers.
Hey… While I may somewhat resemble that remark, I also find it offensive.
I heard a Sky King (B732) coming into MSY and controller issued a direct Jaspo for 19, pilot said ‘negative RNAV’
and then they gave him a vector.
Did any 732 have factory RNAV?
I think WN and DL 732s flew with only VOR/DME/ADF, anybody else?
What about NW DC9? Have they been retrofitted?
NWA’s 752’s don’t have GPS either, though they do have some form of RNAV. I even had a DAL 763 that wasn’t GPS just the other night.
More likely Allegiant. They have a full fleet of MD8s and service LAS pretty heavily.
The OP said that it was AA.
…last night and an American Airlines flight…
I give myself a D for reading comprehension today. I will go stand in the corner now. 