Like other betting sites, FanDuel allows bets to be cashed out under a number of different circumstances, but there are the main things:
The bet must be live - bets that are not yet active, or have settles are ineligible
No major disturbances - this will suspend the cash out option for a while
Wager is on a major sport - sometimes smaller leagues don't have the feature
The game isn't about to end - if the game is about to end, FanDuel will stop allowing cash out