as is stated in the bottom of that message, the reward can only be claimed once. if you didn't et the message, you're either not yet a full member, or you've recieved that reward while being in another brotherhood.

*hugs*

"The reward can only be claimed once" means, among other things, that you cannot claim it again if you did so before in another BH.

If you still think you're qualified for the award, check your BH Achievements tab and see if you have an exclamation mark button next to that achievement. If yes, click it and to claim the reward.