We've seen reports of the Mantle API being unavailable under DA:I's display options despite the fact that the game was designed to support it. This issue is caused by less-than-current Catalyst drivers; AMD has now released the 14.11.2 beta driver that fully supports DA:I.
Please visit AMD's site to download the driver and update your software.
To play Dragon Age: Inquisition, your computer hardware and software should meet or exceed these specifications.
If you have trouble connecting online in Dragon Age: Inquisition, try opening or forwarding these ports to improve your connection.