UNLOCK will appear in the display of the AV receivers if they are not receiving a signal over the selected digital inputs. This can be for a number of reasons;
1) There is no digital connection into that input.
2) The digital input has not been assigned to the source. The optical and coaxial inputs have default locations that may differ from the source you are trying to use. They can be reassigned to the input you want via the onscreen menu's.
3) There is a connection but the amp is not looking at the correct input type- ie the amp is looking for a coaxial connection when the actual connection is optical. This can be changed using the INPUT MODE function.