You're using the Windows standard driver (Microsoft IPP Class Driver). It lacks 2-sided printing.
Fix: Install the Brother Print Support App. Go to Settings > Printers & scanners, select your printer, click 'Printer properties', then the 'Advanced' tab. If you see 'Microsoft IPP Class Driver', click 'Get the app'. The app extends the standard driver's functionality.
If that didn't work, you may be in Windows Protected Print mode on Windows 11 x64. This mode blocks Brother drivers. The Brother Print Support App is the required workaround.
Heads up: 2-sided binding settings are in the app under Basic > 2-sided/Booklet > Settings.