iMovie older version

Can you confirm that you clicked on the Purchased tag at the top of the App Store page.?

Did you change your Apple ID when the newer OS was installed?, if so use the first/original one.

Do you see iMovie listed on the right hand side of the Purchased page ?

Do you know what the version number was for the iMovie that you originally had?

If not what was the original operating system (10.6.x 10.7.x.........10.10.x) that you had as this would help determine

the version of iMovie that the Mac was shipped with.

A lot of questions but you should be able to get iMovie either from original install discs if you started with OS 10.6.8 or from the Purchased page of the App Store if later than 10.6.8 .

Log in to the App Store and go to the purchases tab. Click on iMovie and it should offer you the latest version compatible with your OS version.

That's very strange. Do you have or ever had iMovie on your computer. Do you see iMovie listed under the purchases tab when you sign in to the App Store? The procedure I outlined above should work unless you have never had iMovie before. If that is the case your best bet is to upgrade to High Sierra and iMovie 10.1.9.

That explains why my suggested procedure doesn't work. What you might try is to buy online the iLife 11 DVD suite. This would give you iMovie version 9. Don't know for certain but the App Store might then offer you the latest iMovie compatible with El Capitan. The cheapest and best solution is to upgrade your OS to High Sierra which is free and then install iMovie 10.1.9 which also is free. Hope you get it sorted.