You don’t normally train skill from the inventory window. Skills in your inventory are actually skill books which you have to inject to learn. Once you inject them, they disappear from the inventory but you now know the skill and can train it.
Normally, you use the top of the skill queue window to select which skills you wish to train and drag them to the bottom window to arrange the queue. The skills that disappeared for you may have actually been injected so you can train them like normal. Otherwise, check all your inventory spaces including youth main hanger and those of your ships to see if it ended up there.