If you are not running the correct version of Garageband for the OS you installed then there is no way to get it to run.
What mac model do you have and what year is it. It is possible you could upgrade if it meets the spec for High Sierra.
If you cannot upgrade any further then you will have to search the internet to see if you can download the version
of Garageband that will run on El Capitan.
Just one more thought, have you looked in the App Store Purchased page, is Garageband listed there,
if so and there is a Download button then it may be possible it will download the correct version for El Capitan.