In Thor:Ragnarok, Hela spends a large portion of the movie (off-screen) searching for Heimdal, so she can use his sword Hofund to open the Bifrost.
But we saw in the original Thor movie that Odin's staff can also open the Bifrost; Loki uses it to bring Laufey to Asgard. Hela got the staff pretty soon after getting to Asgard, so why didn't she use it to activate the Bifrost?