As of today, Nintendo seemed to have banned all Legendary Pokemon from being able to be used in a Battle Pass on the Battle Revolution Random Online Matches. This may be due to a large majority going online with nothing but "Ubers" (Legendary Pokemon) such as Mew, Mewtwo, Dialga, Palkia, Lugia and Manaphy. This made the game unfair to trainers using their "Favourite" Pokemon throughout the game. You are still able to use Legendarys in Friend Code battles however along with Single Player.
Hours later a new discovery has been revealed: Nintendo has not placed a ban upon the legendarys, but rather a glitch in the Wi-Fi Connection system prevents your Battle Passes with the legendarys in being accepted. According to reports, if you reset your Battle Pass, it should work but we do not know if this was an intentional thing and this is a way to get round it or whether it was actually a glitch.
I'll edit this or post a new article on any further updates. This article was completely re-written with only minor pasting involved as some members *coughjackattackcough* suggested not to copy/paste the entire article.
Credit goes to DZAPPER for posting info on his thread, Thanks alot DZAPPER! =D
Source:
Serebii.net - Where Legends Come To Life