Per the official Paladins website
, to unlock ranked, you must be at least account level 15 and own at least 12 champions. Since I had already met these requirements, it popped instantly for me post-patch.
CAUTION: I have had reports of this not unlocking for users right away despite their meeting the requirements and even being able to play ranked, so it sounds like the achievement is buggy for some. Read through the comments section for more info on when or how these users finally got it pop (it seems different for everyone).
If you need help meeting the requirements for ranked, see below: