Brother HL-7050N - Manual (Page 154)

ROUTINE MAINTENANCE 5 - 5
HL7050_US_UG_FM5.5
5 Put the new toner cartridge firmly into the printer. If you put it
in properly, the blue lock lever will lift automatically.
Figure 5-8
6 Close the top cover of the printer.
7 If the LCD shows CARTRIDGE ERROR, take out the toner
cartridge and then put it back into the printer again.