I've been running into this issue since YouTube disabled v2 of their data API, and I decided to try my hand at writing a plugin to fix it.
The problem is that some channel pages don't seem to link to their Atom feeds, yet others do. I do not know what causes this.
The result of this problem is that some RSS feeds for YouTube channels can be added to TT-RSS simply by pasting the YouTube channel URL in the Add Subscription box, yet for other channels this will result in TT-RSS (correctly) stating there are no feeds on the page.
The plugin I ended up making is a bit of a hack: Whenever you try to subscribe to a URL like this: youtube.com/channel/<channel id>
(e.g. https://www.youtube.com/channel/UCX20tF ... uOtEyag5EA
) the plugin will translate this into a subscription for the Atom feed by making a tiny snippet of HTML that links to the correct Atom feed. YouTube uses these URLs when you click the channel name below a YouTube video.
It does not work for URLs like youtube.com/user/<nickname>