Well, the pet I want to enter is Tesumi, and I made sure she's well fed, happy and max health but it's still giving me the message "Oh no! None of your pets are eligible for battle. You might want to make sure that they're well fed and happy!" I refreshed the page many times, and I'm not sure what I'm doing wrong, or if this is some kind of error. Any thoughts?

Have you checked your statistics page to see if your pet is stuck in a battle?

There's a Battle Coliseum sticky thread of useful info in the Battle forums, this guide will walk you through everything :)

omigosh I totally forgot about that, that was it thanks so much! :)