Hi! FYI the “Import from Link” feature is the @uppy/url plugin, documentation for it can be found at https://uppy.io/docs/url/.
Maybe the error is there because you try to paste the link to the HTML page rather than the MP3 file? If you paste there the URL of an MP3 file, it should works fine – but I’m not sure SoundCloud provides links directly to MP3 files.
There’s no official plugin for SoundCloud, I haven’t heard of a non-official one either (although I haven’t searched at all to be honest). You could either create one yourself (documentation can be found at https://uppy.io/docs/writing-plugins/), or sponsor someone to make such plugin for you. Depending what API does SoundCloud provide for doing this, it may or may not be easily achievable.