This doesn’t mean that your local files won’t play on Spotify mobile, but simply that you need access to the Desktop app to add the files to your account.
So before you try to sync your files, here are a few things you should know: Getting even one of these requirements wrong will prevent you from accessing the local files sync feature of Spotify. However, there are a few limitations, or rather, prerequisites to consider before you can play local files on Spotify.
Imported audio files can provide a better sound quality than other tracks without having to change your settings and can give you access to tracks not on Spotify, such as fan recordings and more obscure songs by relatively lesser-known artists. Spotify Local Files: The Minimum Requirements