Not sure if Cactuar is an old server or not but are you on a legacy server and was your WAR 50 before in 1.0? Did you do the Job quest to actually learn Inner Beast? If you had a 50 WAR in 1.0 the skill will show up on your hotbar but it won't be usable until you actually do the quest to unlock it. Same thing happened on my DRG with Jump and Phlebotomize.