Thanks for reaching out to us about this! We hope you don't mind us replying in English 🙂
first, could you let us know if the song that you are trying to move to your playlist is a local file or a track from the Spotify catalogue?
Do you also mind making sure that the 'Show unplayable songs' setting is toggled on in your app's settings, and if toggling it on makes the song reappear in your playlist?
Keep us posted 🙂