I cant sprint.

I believe default sprint has a toggle between clicking and double pushing the left stick - make sure it is set to how you want and to do it from the main menu since custom mapping in a match can cause issues.

There are also cooldowns in certain situations so try testing it out on a road or other flat pathway - sometimes it's as simple as just standing still for a split second and then continuing on. I haven't heard of a game error causing a player to stop sprinting but you can always try reinstalling the game as a last resort as well.