Check the following points to resolve the issue:

If you are using a non-genuine Brother toner cartridge, the machine may or may not detect the toner cartridge. 

Install a new toner cartridge.

The toner cartridge model number is not compatible with your machine.

Verify the cartridge model number will work with your machine.
Visit the Online User's Guide and Consumables & Accessories section on this web site to know what consumables are available for your machine and its toner cartridge model numbers.

> Click here to see the latest Online User's Guide.
> Click here to see the Consumables & Accessories section.

The toner cartridge or the toner cartridge and drum unit assembly is not installed correctly.

Remove the toner cartridge and drum unit assembly. Remove the toner cartridge from the drum unit, and reinstall the toner cartridge back in the drum unit. Reinstall the toner cartridge and drum unit assembly back in the machine.

> Click here to see how to reinstall the toner cartridges.

If the error message is not cleared, contact Brother Customer Service from the "Contact Us" section.