how fix directx mantle for battlfield 4 pc

Hi @micheall1234 and welcome to Answers HQ!

There's no such thing as "directx mantle" - there's DirectX and there's Mantle. Two different API's.

If you've set the game to Mantle and it now can't start you can change it back in the settings file located at:

C:\Users\YOURUSERNAME\Documents\Battlefield 4\settings in the file PROFSAVE_profile - just set GstRender.MantleEnabled to 0 instead of 1. Alternatively you can also delete the entire settings folder to reset your game's settings completely.