I recall seeing that message when a channel was temporarily off-air.

Assuming that some of your channels are OK try redoing the channel scan.

An extract from the TiVo User Guide:

"Channel scanning identifies all the channels that the antenna is currently receiving. Signals
received via antenna are subject to weather conditions and other environmental factors, so the
availability of certain channels may vary. This means that you may see channels appear and
disappear from the channel list each time you perform a channel scan."