The DECODER OFF message is purely an information message. It can mean one of two things - a two channel source is input or there is no input. In either case the receiver would not need to turn on a decoder to play sound. You can toggle this informational display by pressing the INFO button on the front panel of the receiver or on the remote control (depending on your model receiver) – you will have three choices: DSP Program, Audio Decoder, and Input.
The INFO button basically tells you the status of the receiver. You cannot "turn the decoder on" - the receiver is designed to automatically select and activate (turn on) the proper decoder based on the input signal from the source (DVD player, BD player, cable box, etc.). For example, if your DVD player sends a Dolby Digital signal to the receiver the receiver will see that and automatically activate the Dolby Digital decoder.
If the decoder off message is accompanied by a loss in picture and/or sound, we would suggest double checking all your connecting cables between the source component and receiver. Just one bent or damaged cable or connector can cause a loss in picture and/or sound.