Like the title says, I've tried multiple times in Bozja to complete this quest but there is no one attempting Castrum, it's impossible to beat with the 8 or fewer people who sign up to do this.
It's a pain since it's impossible to move the resistance relics past step 3 until you complete Castrum at least once.
Please give us another way to complete this quest if there aren't enough people running Castrum to give people who need a clear a decent sized party. It's been this way since Endwalker released with no sign of changing.