How can I find the macOS version codename?
The version number is the correct thing to report when you have issues. You should also include the subversion.
The marketing names are from macOS - Wikipedia article, up to and including macOS Catalina.
╔════════════════════════════╦════════════════════╗
║          Version           ║      Codename      ║
╠════════════════════════════╬════════════════════╣
║ Rhapsody Developer Release ║ Grail1Z4 / Titan1U ║
║ Mac OS X Server 1.0        ║ Hera               ║
║ Mac OS X Developer Preview ║                    ║
║ Public Beta                ║ Kodiak             ║
║ Mac OS X 10.0              ║ Cheetah            ║
║ Mac OS X 10.1              ║ Puma               ║
║ Mac OS X 10.2              ║ Jaguar             ║
║ Mac OS X 10.3              ║ Panther            ║
║ Mac OS X 10.4              ║ Tiger              ║
║ Mac OS X 10.5              ║ Leopard            ║
║ Mac OS X 10.6              ║ Snow Leopard       ║
║ Mac OS X 10.7              ║ Lion               ║
║ OS X 10.8                  ║ Mountain Lion      ║
║ OS X 10.9                  ║ Mavericks          ║
║ OS X 10.10                 ║ Yosemite           ║  
║ OS X 10.11                 ║ El Capitan         ║
║ macOS 10.12                ║ Sierra             ║  
║ macOS 10.13                ║ High Sierra        ║  
║ macOS 10.14                ║ Mojave             ║  
║ macOS 10.15                ║ Catalina           ║  
║ macOS 11.0                 ║ Big Sur            ║
╚════════════════════════════╩════════════════════╝