What browser are you using? The version of Safari used with macOS 10.11 has not been updated in many years and won't work with many websites since the Internet has changed a lot in those years. Try using a third party browser such as Google Chrome or Vivaldi. Unfortunately Firefox is no longer supported on macOS 10.11. If your Mac cannot run a later version of macOS, then I recommend using Vivaldi since it is more likely to support macOS 10.11 for a longer time than Google Chrome. Vivaldi uses the same Chromium web rendering core as Google Chrome, but includes lot more configuration options and features.
If you provide us with the exact model of your Mac we can see if it is possible to upgrade to a newer version of macOS. What is the exact model of your Mac? You can get this information by clicking the Apple menu and selecting "About This Mac" or by entering your serial number here or here.