Your pets need to be fed, happy, and have more than 0 health in order to battle.

Make sure that you aren't frozen - if you're frozen, you can't select weapons. If you still have the problem, please start a new thread for that!

As I and others have already stated, you need to make sure that your pet's health isn't at 0. Based on your pet's profile, your pet PrincessMiMi has the following health: HP: 0/6,552

You need to heal your pet before you can battle again.