In basketball, a great coach can’t make you the best team in the league all on their lonesome. Just like a bad coach can’t make you the worst team in the league on their own. At the end of the day, players are the ones who do the majority of the winning/losing, not the coaches.
However, a great coach can unlock something special in their group and extract a couple more wins out of them than otherwise thought possible (for example, Erik Spoelstra and the Miami Heat). Meanwhile, a bad coach can keep a great roster from reaching their ceiling (we’re not going to cite a specific example because that would be unnecessary).