Canonical => !no target specified!

When I run hugo check I get a bunch of errors/warnings (no idea which) like: (renderer: markdown) (layout: page/single.html, exists: false) (layout: _default/single.html, exists: true) (layout: theme/page/single.html, exists: false) (layout: theme/_default/single.html, exists: false) canonical => !no target specified!

Can somebody please explain to me what this means? Also if any of the developers are reading this, maybe some less confusing error messages would be a good idea? Thanks.

This isn’t error messages. hugo checkis more of a “debug tool”, and shouldn’t normally be needed (I’ve never used it).