Liked Videos and Liked Music are private playlists, they have a generic URL and you can only access them by being logged in. I have two methods to download Liked Videos:

• Install a desktop app video downloader, most of them have the option to download an entire playlist.

• They also have an option to add a cookies file so you can download private playlists, such as Liked Videos or Watch Later

• Log in to YouTube in your browser

• Export the cookies from the browser

• Import them to the desktop app

• Download Liked Videos

That's what you're supposed to do, however i never got that to work. If it doesn't work, you can transfer all your liked videos from the private playlist to a public playlist. Here's what i do:

• Create a new playlist called Downloads, and make it public; it now has its own specific URL, unlike the generic URL of Liked VIdeos

• Install the YouTube Multiselect browser extention

• Use Multiselect to select every video in Liked Videos and add it to Downloads

• Use the desktop app to download the Downloads playlist

This is how i download my private playlists like Watch Later or Liked Videos, i assume it also works for Liked Music