The problem is that the Apple I.D. of the prior owner is different than the one you are using. You can update a version of iMovie that was acquired with your Apple I.D, but not a version that was acquired with someone else's Apple I.D.
Perhaps if the prior owner is a friend of yours, the prior owner can use its I.D. to acquire iMovie on your computer. Or, your could upgrade your operating system as the error message said.
Check your Applications folder to see if maybe the iMovie app appears there. If not, you probably are out of luck.
-- Rich