OS X El Capitan version 10.11.6. Macbook Pro, Late 2013 doesn't show Big Sur update

Howdy,

See if you can download it from the App Store and install from there.

From How to upgrade to macOS Big Sur

Download macOS Big Sur

If you're using macOS Mojave or later, get macOS Big Sur via Software Update: Choose Apple menu  > System Preferences, then click Software Update.

Or use this link to open the macOS Big Sur page on the App Store: Get macOS Big Sur. Then click the Get button or iCloud download icon.

ivan

Don't even bother to download Big Sur until you have 45.5 GB of available storage (beyond the 12.2 GB installer) that it requires to upgrade from El Capitan. Put your MBP on a power supply, and a solid network connection. Be patient.