Cash-out may not be offered on every market that you like to bet on. Usually cash out is very clearly marked when available and on some occasions, it will be stated that cash out is not available. A good example of this is on Betfair sportsbook, where cash out is almost always available on win bets on horse racing but almost never on the place betting markets. You can reply on Betfair for cash out unavailable to coincide with markets that do not go in play on the exchange