iMovie doesn't work

So first, I will start off by saying that all of my software is up to date and all apps have been updated as well, including iMovie. But every time I try to open iMovie it says "This version of iMovie isn't compatible with this version of Os x (You are running 10.3)". It doesn't even make sense that it would be saying this, I just updated everything so I don't know whats going on or why its saying I'm running 10.3 even though I know I'm not. Can someone please help me, Thank you in advance!