i think i understand your question
when you first deposit and take a sign up bonus , your money is locked and can’t be withdrawn until you have turnover xxx amount in the casino on games that qualify. (this is the meaning of WR = work rate)
many of the casino’s ban blackjack for qualification (roulette as well normally, but generally ban games with low house edge) . normally the terms go something like this
“deposit $100 and receive $100 sign up bonus. the deposit and bonus need to be turned over 40 times before being able to be withdrawn. Play on blackjack, craps, bacarat and roulette do not count towards the turnover”
in past years the sign up bonuses were excellent and mathematically +EV , for example blackjack has a house edge of less than 1% , so in theory , playing a $100 deposit/$100 bonus (so $200 to start with a 20x requirement ($4,000 work rate) with a house edge of 0.5% at $1 a hand flat betting would result in a loss of only $20 at the completion of the terms of the bonus. so deposit $100 and withdraw $180 .
obviously this is a no-win situation for the casino , if too many advantage players take up their offer.
so now , most casinos offer sign up bonuses that restrict the work rate to games that have a high house edge and a high work-rate……most bonuses are mathematically -EV for the player, which means by the time they can withdraw their money, they should have mathematically lost it anyway, unless they have a bit of luck.
the short answer to your question, is you cant withdraw the bonus or your deposit money until you qualify under the terms of the bonus
hope this is clear enough
cheers