ERROR: "YouTube is unavailable. Please try again later" is displayed when I press the Power button or YouTube button on the remote control from standby mode.

Select CANCEL on the error screen to dismiss the error message. When the error message is dismissed, the YouTube home screen is displayed.

This error may occur when turning on the TV from standby mode because the TV takes about 3 seconds to reconnect to a network.
After dismissing the error message, you can select playback content as usual.
However, if the TV does not have internet access, it may not be able to play back YouTube content.