You’ve probably noticed that some of the skills are greyed out and you can’t upgrade them.
This is because you’ve chosen to ‘specialise’ your cat in another skill – so for example if your cat is a pouncer you won’t be able to update the protection skill. If you’d like to have a cat with two skills you need to use the cat merger to merge with a cat that has that skill already (e.g. if you merge a cat with an attack still with one with defense the new cat will have both)