When a brother printer is not printing the documents and keeps showing the toner not detected error. you should check these settings on your device.
1. first of all, make sure that the toner is in the working condition.
2. Go to the settings and reset the toner cartridge.
3. Make sure that the printer is using the branded cartridge only. if you are using the old cartridge, you need to remove the current cartridge from the printer and then install a new toner on the brother printer
4. Let's update the brother firmware on the device.
Once you will make these changes, your device will start working fine. in case, if you are still getting the drum error. you need to visit: How to replace the drum on the brother printer?