If you have a bunch of JP for your Squire (let's say 500), you should have no problem at all spending a couple hundred for Counter Tackle. If something's grayed out, you don't have enough JP for it. Simple as that.

If you're talking about the Monk's Counter ability (since you're not specific), remember that JP are specific to jobs. If you have 3,000 JP for your character as a Squire, that 3,000 ain't gonna carry over to when he's a Knight, Monk, Chemist, or any other job.

Lastly, if your guy is knocked out but not a crystal or treasure chest when you finish a battle, he will live.