Some bonuses may stipulate a turnover requirement. This is the amount that has to be bet using funds in the bonus wallet before any winnings generated from the bonus become withdrawable. All bonus winnings are kept in the bonus wallet until the turnover requirements are complete.
If your selections are eligible for the free bet you received, the 'Use eligible bonus' option is presented, tap the button (on the sportsbook app) and the 'Toggle' (on Desktop/ Browser or the Exchange app) to enable use of your free bet.