Under "Default settings" section, you can find "Comments on your new videos." Beside it, you can see "Disable comments" option. Click on it.
Click on "Save" button at the top.
Note: If you want to allow comments in new videos in future, just go the "Comments on your new videos" option. Click "Allow all comments" or any one of the other options you prefer and then click "Save" button.
Disable Comments on Your YouTube Channel
You can disable comments on your YouTube channel and here's how: