See the echo naming quirks thread. It seems pretty clear that some words are reserved for echo’s own use, perhaps for future functionality. TV does seem to be one of these, as is ESPN.
If you use the IFT TT channel method so that you were saying “Alexa, trigger TV,” it will work. Once you put the “trigger” in there it will pretty much let you use anything.
But if you’re using any of the other methods where echo has to parse what you’re saying to figure it out, you just have to work around these quirks and choose a different word.
At our house we ended up using “local” And “cable” depending on exactly what device we wanted.