How can I enable Brother printer's two-side printing functionality? My Brother DCP-7055 Printer’s two-side printing functionality does not work (the “Two-Sided” option is greyed out) on macOS Catalina 10.15.4. I have not tested it on any other versions of macOS but it works as expected on Windows. I've installed the macOS driver from the official Brother website, and the installed driver is Brother CUPS as it is stated by Brother. Is there a workaround to enable this option of the device?
On the page you linked, on the Printer driver download section, it specifically states:


  *Not all features are available for all products.


This is one of those features

Also on the Supported OS page, it likewise states:


  Please note that the available features may vary depending on the operating system.


It appears a Brother is using a generic(ish) CUPS PPD file meaning it’s a one-size-fits-all driver and two sided printing is simply not available. 

It’s not a “popular” piece of advice, but your best bet is to get something with a bit more robustness in driver support like HP, Xerox, or Lexmark.  Brother has been really strong in the Windows space, not so much with Mac.