Untitled Flash solo movie is now slated for 2021 release.
According to Variety, Warner Bros has pushed back the filming for movie. The movie was expected to start in March but the script is still being worked on. Warner Bros believes the script may not be finished in time.
Another reason for the push back is the third Fantastic Beast movie (another Warner Bros property and Flash‘s Ezra Miller plays Credence Barebones, a major supporting role). Filming for the third Fantastic Beast is scheduled to start in July which will clash with Flash if it begin to film in March and create scheduling conflicts for Miller.