The only way to get iMovie 10.1.6 for your El Capitan 10.11.6 is to get iMovie listed on your Purchased page in the App Store.
The only way that I can think of doing this is with the help of a willing friend of yours that owns a Mac running 10.13.6 or later.
I have not done this myself but believe that it should work.Nothing that I am putting here is illegal.
On your friend's Mac drag his/her iMovie application to the trash but do not empty the trash.You will drag it back out later.
Still on their machine, you sign into the App Store with Your Apple ID.
Now download iMovie as you attempted before.It should as it is on a Mac with OS 10 .13.6 or later.
You should get a .dmg file .
Drag this .dmg file to the trash as you are not going to use it on this machine.This process though should place iMovie onto your Purchased list.
Do not empty trash.
Drag out of the trash your friends iMovie App that was put there earlier and place it back into the Applications folder.
If you wish you can now empty their trash but not essential.
Their Mac is now back as it was.
Now on your Mac sign into the App Store with your Apple ID and click on the Purchased tag at the top.
You should see iMovie listed and against it it should say Install.
Click Install and you should be offered a version that is compatible with your OS.
It will not say what version it is, but if it downloads it will be compatible with your 10.11.6 .
If you do have a friend who can, and is willing to, help let us know how you get on.
Have your mobile phone to hand when doing the above as Apple may send you an authentication code when you sign into the App Store on your friend's Mac.
