I've done that one and just completed the next one that has 4 champs with 6 lvl 16s

When i did the one you're on, i found that i did not meet the requirements.  Sometimes the numbers merge with the pictures of the gear and they can be hard to read.  I went through every champ that i had put some lvl 16 gear on and found that 1 had 1 piece of lvl 12 gear.