Refer
here for the channel ID.
If there are multiple entries, please provide a list of channel IDs separated by comma: (e.g. UCK8sQmJBp8GCxrOtXWBpyEA, UCa123456789012345678901)
The entered Channel ID is invalid. Please enter an ID in the following format: UCK8sQmJBp8GCxrOtXWBpyEA