YouTube has a number of ways to access a channel.  Sometimes the username works, sometimes not.  The only consistent method is the channel_id. 

YouTube instructions to find your channel ID. 

Find your channel ID

You can see your channel's user and channel IDs in your advanced account settings.

  1. Sign in to YouTube.
  2. In the top right, click your profile picture and thenSettings Settings.
  3. From the left Menu, select Advanced settings.
  4. You’ll see your channel’s user and channel IDs.