The paper size specified in the printer driver is too small for the machine to deliver it to the face-down output tray.

Open the back cover (back output tray / face-up output tray) to let the printed paper out on the back output tray / face-up output tray and then press Retry, Go, Black (Mono) Start or Color (Colour) Start.

NOTE: Illustration shown below is from a representative product and may differ from your Brother machine.