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We're sorry to hear that you're not able select TV input. If a particular input can't be selected or greyed out, it means that there is no device connected to port. Are you using an over-the-air antenna or a cable box? Most cable boxes use HDMI input. When using an antenna, an auto program needs to be done to be able to select TV input.
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I have the same issue. I am trying to select "Antenna" as input for watch TV but it is not listed. It is connected. I can in fact watch antenna TV by opening the "Google Play and TV" App and hitting the Guide button and selecting a channel. However I cannot connect the "Watch TV" icon to the proper input.
I have a 55x810c and recently installed a software update.