While I feel bad for everyone involved, i'd like to share some quotes from various sources that saw a version of the movie and basically why it was cancelled. As I said in the HBO Max thread, it was basically cancelled to save the Batgirl brand from her being seen as a lesser-than superhero.
Let's go down the line with some things that can be found that are said about the movie:
Those tests were said to be so poorly received by moviegoers that the studio decided to cut its losses and run, for the sake of the brand’s future. It’s a DC disaster. “They think an unspeakable ‘Batgirl’ is going to be irredeemable,” the source said. - NY Post
Spoke to people that saw Batgirl and they said it was a huge disappointment. Also that the costumes looked cheap especially Keaton’s Batman costume. I figured WB would dump it on HBO Max but looks like we will never see it. - Collider
The word for awhile with insiders was that the film (Batgirl) was beyond salvageable. The question of if it would do more harm than good to the brand has been raised several times. - Former THR Editor
Let's add on to that the tax writeoff for this will probably cover at the very least whatever the movie would have made at the box office since it probably would've been a low-grossing film, and since they aren't releasing it, they don't have to spend the money to market the movie meaning they are potentially saving money by not releasing this movie on top of protecting the character/brand from being ruined with a bad film.