Try right clicking on the channel number, then select "delete".
There must be a valid receive frequency at a minimum for the channel.
If you no longer need that channel, try as I suggested above.
If you want to change the receive frequency on a channel, simply highlight it, then over write with the new receive frequency.
If it is a receive only channel, highlight the transmit frequency, then hit the backspace on your keyboard, then hit the enter key on your keyboard.
Edit:
This is not the proper forum to be asking about transceivers, This should be in the Kenwood forum. I have reported, and asked that the Mods move it.