I can't add music to my new slideshow

What happens when you try to add music?

Are you creating the slideshow project as described here? See: Create slideshows in Photos on Mac - Apple Support

To use your own music,

• click the Music button in the sidebar.
• Clear the predefined theme music in the "selected Music" brick.
• Search for the music you want to use in the "Music" brick. You cannot use songs downloaded from Apple Music, only songs without digital rights attached, for example Audis files you recorded yourself, songs ripped from CDs, or similar.
• If the song you want to use is dimmed and cannot be selected, laugh the Music.app and assure, that the song has been downloaded from iCloud by downloading it and playing it.