Why can't I purchase certain champions and skins?

Occasionally, I will get a skin permanent from the chests you unlock and attempt to unlock the champion, but only to receive an error saying I can't purchase the champion. This has happened with a Scorched Earth Xerath skin (which I disenchanted) and a Shadowfire Kindred , which I would like keep but I can't purchase Kindred. Also, I wanted to see how much champions on free rotation costed as I was looking into Lulu and Ivern , but it also said these champions were unavailable for purchase, which greatly confused me. Can anyone explain why this is happening to me? I'm level 32 so I'm not sure why this is. Also I apologize if I posted to the wrong board I didn't know where this fit in.