Gellert Grindelwald: Our war with the muggles begins today.
Albus Dumbledore: The world as we know it is coming undone. Things that seem unimaginable today will seem inevitable tomorrow. Should you agree to do what I ask, you'll have to trust me, even when every instinct tells you not to.
Answer: Dumbledore, as a Hogwarts professor, did not have any legal authority to apprehend Grindelwald or anyone else. That was up to the magical authorities. Also, this was meant to be a four-part (and possibly five-part) movie series that continued the story leading up to Dumbledore and Grindelwald's final duel. The third movie underperformed financially, so the further sequels were cancelled, leaving the storyline incomplete.
raywest ★
But in the Crimes of Grindelwald, Travers - Head of Magical Law Enforcement for the Ministry personally asks Dumbledore to fight Grindelwald. Then immediately after in the film loads of people attack Grindelwald including Kama who doesn't work for any magical authority?
Kama and the others made their own decision to do that, whether or not they had the authority. Dumbledore obviously felt it wasn't his responsibility to apprehend Grindelwald or lacked the will, particularly considering their mutual history.
raywest ★