This means that the guide data you’ve selected doesn’t have information for that channel. If the channel is from a neighboring market which you are able to receive that might explain why.
The best thing you can probably do would be to report the missing guide data using the information from this thread (you can find your guide/lineup name at the top of you Live TV & DVR settings in Plex):
And, potentially follow that up with a quick post over in this thread noting what’s missing and that you’ve completed the form:
Also, a new EPG provider is going to be used in the (hopefully) not-too-distant future. So, this problem may self-correct.