I got this error with 15.1 and i had around 4 GB of space left after downloading the update, i deleted some apps so i had 10GB available and then i could install the update.
siggibo
Users receive reputation points by contributing quality content to the forums.
Posts include post and replies published on the forums.
Post authors can mark replies as Accepted to indicate successful solutions.
Apple Developer Forums admins can mark replies as Apple Recommended to indicate an approved solution