In the film adaptation of the fourth Harry Potter book, the fictional band in the Yule Ball scene - despite being named in the book - goes unnamed in the film. Why?

The band in the book were named the Weird Sisters (despite being an all-male band). However, there was an existing Canadian band named the Wyrd Sisters (all female), who owned the rights to that name in Canada. According to Wikipedia, Warners offered the band first CAD$5,000 and then later CAD$50,000 to use the name in the movie, with the band rejecting both offers -- so the movie omitted the name. But the band then sued Warners for CAD$40 million . . . even though the name didn't appear in the movie and so there was no real basis for the lawsuit. However, naturally the lawsuit was dismissed, and the band was then ordered to pay Warners CAD$140,000 in legal costs for bringing a baseless lawsuit. According to Wikipedia, both cases were eventually settled out of court in a confidential settlement (which, although not revealed, likely means that the Wyrd Sisters didn't end up having to pay the judgment against them).

https://harrypotter.fandom.com/wiki/Weird_Sisters
https://en.wikipedia.org/wiki/Wyrd_Sisters_(band)
https://en.wikipedia.org/wiki/Legal_disputes_over_the_Harry_Potter_series#The_Wyrd_Sisters
https://web.archive.org/web/20070917053558/http://www.canada.com/nationalpost/news/story.html?id=bc5b3049-56dc-493b-9ccc-5d4bd0389392
https://web.archive.org/web/20070507075524/http://www.cbc.ca/arts/story/2006/07/03/wyrd-potter-court.html
https://web.archive.org/web/20110714174133/http://www.news919.com/news/national/article/39866--wyrd-five-year-court-battle-over-harry-potter-movie-ends-with-secret-settlement