You must have 2 video cards, or you wouldn't be seeing the message, correct?
Some cards don't work as a secondary card, and must be set as the primary or
used alone, or they won't work.
You might try setting the prophet as the primary and either disable your
onboard (if you have it) take out the other card (if that's the case) or see
if the other card will work as the secondary.
As far as how to change the card, it's in your bios. Actually, it may or may
not be there.as it depends on the motherboard.?