Your current version of GarageBand 6.0.5 should be able to run on OS X El Capitan 10.11.6. Try to find out, why it is no longer working. I have been using GarageBand 6.0.5 on all newer system versions up to 10.14 Mojave, until Catalina killed it.
Ii is no longer possible to download the older versions of GarageBand 10. You have to grab the applications during the short window of opportunity, right after they have been released, when they are available for your system version.
What happens, when you try to launch GarageBand 6.0.5? Is there an error message? The most common reason is, that a project got corrupted, and garageBand is crashing when trying to open the most recently used project. Try to move your last project to a different volume, or to launch GarageBand by double-clicking a different project.