Although Barry Allen was comfortable enough using his powers to foil small time crime and ambush average criminals, he was nervous about entering real combat against Steppenwolf and his Parademon army.
Batman advised that Flash just focus on saving one person and go from there, and that did the trick.
By the end of the movie, in addition to having gained a job at the Central City Police Department, he seemed more comfortable in his role as a swift protector of humanity.
Several months after Batman v Superman’s release, this in-costume Flash cameoed in Suicide Squad to apprehend Captain Boomerang, but it was almost a year and half later that he finally made his first full appearance in Justice League.