Since updating to mac OS Big Sur 11.2.1 I can't connect to my printer "Brother MFC-8420". Please, what is the fix?

Downloads | MFC-8420 | Others | Brother

brother does not support a higher version of macOS than OS X Yosemite, it's probably a 32bit driver so it would not work

you can try contacting brother

or you can try downgrading to what macos version it worked on

or you can get a new printer

Hi,

Under which operating system version was this printer last working, before the update to macOS Big Sur 11.2.1?

Before getting a new printer, there may be a few things to try with the existing one.

First of all, you may want to just reset the printing system and try to re-add the printer.

Reset the Mac printing system to solve a problem - Apple Support

Add a printer on Mac - Apple Support

Does the operating system then automatically offer a suitable driver to use?

When a OS X 10.10 Yosemite driver does not work with macOS Big Sur, it can sometimes be worth testing macOS Catalina 10.15 drivers (for other models, perhaps something like the random example https://support.brother.com/g/b/downloadlist.aspx?c=us_ot&lang=en&prod=mfc9120cn_all&os=10060). If so, that would be purely experimental. It may not work at all. There is always a certain risk involved with any experiment. Software conflicts cannot be ruled out. You may have to erase and reinstall everything on the computer. Data can be lost. A hardware problem is unlikely, but it is impossible to guarantee anything. So, you must know exactly what you are doing. Also, if you decide to carry out tests, make absolutely sure that you have multiple backup copies of important files first.

Furthermore, the MFC-8420 specifications indicate that the printer is capable of PCL6 and PostScript emulations. So, if nothing else helps, you may even want to try (macOS) generic drivers. If necessary, via a network connection provided by an external Ethernet-to-USB (or -parallel) print server connected to one of the LAN ports of a router (see the Add a network printer by its IP address section in the Add a printer on Mac article).

I suggest you get an AirPrint capable printer so you don't need to worry about drivers ever again. This Apple document lists AirPrint printers: About AirPrint - Apple Support