https://variety.com/2018/film/news/flash-standalone-movie-release-date-1202980356/
Warner Bros. is pushing back the start of filming on the studio's still untitled standalone Flash film, sources tell Variety. The film never received a formal greenlight nor did it have a set release date, but it had been expected to begin shooting in March. However, a script is still being tweaked and the studio thinks that it won't enough time to get the script in shape in order for the film to make its original start date.
That's a problem, because Ezra Miller, the live-wire performer tasked with playing the Flash, also has a key supporting role in "Fantastic Beasts," the "Harry Potter" spin-off series. The third film in the franchise begins shooting in July, which would cause scheduling headaches. The standalone Flash film is now expected to commence production in late 2019. That likely means the superhero adventure won't debut in theaters until some time in 2021.