The duplex unit on your printer can be set to work with either A4 or Letter sized paper. If your PC software is not configured in the same way as the lever position, you will see the DX LEVER ERROR message when you try to print in 2-sided(duplex) mode. Check that your printer driver and software is set to the same position as the lever, or if the lever is not set correctly, change its position. Please note that the lever is located to the rear of the printer just below the silver serial number sticker.