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You should be able to just disable the internal speakers and that should turn on the optical output.
The reason you were getting feature unavailable before is because the tv is looking for a viera sound bar or surround system connected via hdmi, when it doesn't find one, it disables that option.
Can you tell me the theater system you are using?
Ideally, you shouldn't even need the optical cable from the tv to the receiver, as you'll get the best connection by running everything into your a/v receiver over hdmi, with a single hdmi out ot the tv. This will keep all sound in the receiver, and just pass video off to the tv.
The only reason not to do this is if your receiver doesn't have enough of the correct inputs, or if you're using an antenna with the built in tv tuner and just need to get that audio out to the receiver.
Thanks,Went and purchased
a HDMI cable, connected to HDMI 1 on tv and HDMI out on receiver of the theatre system. All good now.