Thank you for contacting us. Theoretically that line of code should work, so I need to ask you some questions to determine what the cause may be.
Firstly, what is the contents of your GetOctopusClient() method? Do other methods on the client work properly? How are you retrieving the channel?
Also, there are repository methods available for Channels that may prevent these errors and simplify your usage. For example, you can substitute the line above with:
GetOctopusRepository().Channels.Delete(channel);
And in this instance the client will determine the correct URL for you.