Why Am I Not Allowed to Unlock a Champ When I Can Afford It?

Hello! I just started playing last week so I apologize is this has an obvious answer, I did google it but I didn't find anything maybe I googled wrong. xD

Basically I want to unlock a champ I can afford (Ashe) because I like her but lost her when I got a new round of free to play champs. But when I try to I get an error that says something like "Error, you cannot unlock this champion at this time" or something like that. I tried with both ways of unlocking and I get the same error. I also went to the buy more (rp?) points and I got an error page. I have restarted LoL many times and every time I get the same error. What am I doing wrong? I thought maybe I'm not high enough level (I'm 7) so I wasn't allowed to unlock maybe yet?

Thanks for your time!

~ God Bless ~

`S'Mokona