Hi, flyguy1982,
You would need Catalina to run the most current version of iMovie. That's why you are getting the "incompatible" message.
Try upgrading Yosemite to as high as you can go. Then try this procedure, that assumes that you had iMovie 10 on your computer sometime in the past. Even if you haven't, try the procedure anyway.
To update/redownload iMovie 10, drag your iMovie 10 app from the Applications folder into the trash, but do not empty the
trash. (You can skip that step if you had iMovie 10 on your computer before, but deleted it.) Sign in to the app store and open your purchases folder by clicking on your name at the bottom of the app store side bar. (NOTE: For El Capitan and older, click on the Purchases tab at the top of the app store screen and then sign in. Click on Purchases again to reveal your
purchases.) You will see your purchases displayed, including iMovie 10. Redownload/update iMovie from there. If not
successful, drag your present iMovie app out of the trash (if you moved it there) and return it to the Applications folder.
-- Rich