Have you downloaded the driver from this page?
http://support.brother.com/g/s/id/os/macintosh.html
There is a note, that the Sierra drivers cannot (yet) be installed on Sierra, but if they already had been installed on El Capitan, they will work. Brother announces a fix to be released in November:
Mac OS X 10.11 (El Capitan) drivers cannot be installed on macOS 10.12 (Sierra). However, in most cases, drivers already installed on Mac OS X 10.11 can be used when updating to macOS 10.12. Currently Brother is verifying that Mac OS X 10.11 drivers will work with macOS 10.12.
At the late November Brother will release a new Full Driver & Software Package for products which already have the Full Driver & Software Package.