Temporarily set your Mac’s system date back to 2016 in System Preferences. Run the installer, then set the date back to "Automatic" once the installation is complete. Performance Tips for Older Macs
GarageBand is a free download from the Mac App Store. To download GarageBand on Mac OS X El Capitan 10.11.6, follow these steps: garageband mac os x el capitan 10116 download link
Keep in mind that GarageBand on El Capitan (10.11.6) might have some limitations and known issues, such as: Temporarily set your Mac’s system date back to
Sometimes, users might have a very old version of GarageBand (like GarageBand '11) installed. The App Store might try to update this rather than install the newer legacy version. If the update fails: such as: Sometimes