Cannot join alliance war

Dragster8

It keeps showing error whenever I try to join the alliance war, also happening with one of my alliance member

SummonerNR

You are trying to join a different BG for attack than the one in which you have placed defenders.
-or- you are trying to use champs that are in either of the +/- “potential” Tier Maps (even if they are not the defenders that are being used in your “current tier map”, the other Tier ones can also NOT be used for attack or for AQ).