To download a version of GarageBand compatible with Mac OS X El Capitan (10.11.6)
Before downloading GarageBand, ensure your Mac meets the system requirements:
What is GarageBand?
OS X El Capitan 10.11.6 , the officially compatible versions of GarageBand are GarageBand 10.1.2 or the older GarageBand '11 (version 6.0.5)