Welcome to FlyerTalk, faloppi!

I think there may be some confusion here on what upgrade inventory is relevant to your trip. If you're on a 2-class flight, then you are correct that you need A2 inventory for your award upgrade with your companion. However, if you are on a 3-class flight, then you need C2 inventory to upgrade from Economy to Business.

(Next time, if you can provide more complete info up front, such as the routing and flight number or equipment, that would help )