Williams is a good and valuable player and I'm sure Bilbao wants to make him stay however he can easily leave the team if he gets a good offer from the other team and this makes the fans of Bilbao angry because they can't keep their superstar player and Williams is very close to join Barcelona and play there for the next season. Even Williams himself likes to play for a team like Barcelona and this made everything harder for Bilbao to keep him there.
His performance was good enough while playing for Spain to make other teams send him an offer too.
Williams' contract runs until 2027, so Bilbao could theoretically keep him for another 3 seasons, but I don't see any information anywhere about what the release clause is in this contract. If it is 50-60 million or something like that, then Barcelona can simply buy him out without the consent of Bilbao, but if there is a serious amount, then the negotiations will be long and Bilbao will try to maximize their earnings.