where can i download osx 10.11?
MacBook Pro
Check to make sure your applications are compatible.
El Capitan 10.11 Compatibility information
macOS Sierra 10.12: Compatible Apps
Also check to make sure there is a compatible driver for your printer.
Do a backup before installing, preferable 2 backups on 2 different drives.
Ok so Mac OS X El Capitan version 10.11 is no longer available for download, However the last version of El Capitan is and that version is 10.11.6, you can get it via the App Store, read this, How to upgrade to OS X El Capitan - Apple Support
You haven't said what model and age your Mac is, if your Mac is capable of installing an OS newer than El Capitan then you will be refused the download of El Capitan on that Mac. In that event you should be able to download and install High Sierra.
How to upgrade to macOS High Sierra - Apple Support
how to install el capitan osx 10.11
