I am unable to download 10.6

Hi there Apple community!

I'm trying to update to the Os Maverick system because I'm having difficulty accesing some web content, especially stuff like Youtube. Since I primarily use my computer for Photoshop, I don't tend to update the OS all that often. Currently, it's at 10.5.8. I haven't been able to update to 10.6 mostly because until now I didn't have any performance issues. I'm working on a Mac Pro purchased in 2009, it's 64 Gb and usually doesn't give me problems. It's only the past few months it's been acting up like this.

I have read that this model can be updated to the Maverick 10, but every time I try to upload a new Os system, the computer tells me I need the version that existed prior to this. Currently, I just downloaded OS 10.6, the version that immediately preceeds the OS I am using, and the computer is claiming I still need OS 10.6 in order to run it.

Any ideas guys? It's pretty vexing. :/