Why can’t I access my music library from my photos slideshow ?

On a Mac you can access your Music Library from a slideshow. But there are some limitations, depending on the system version on your Mac.

• Basically, the song you want to use has to be stored locally on your Mac, so ensure the song has been downloaded from iCloud before you try to select it.
• And the song must not be protected by digital right. Depending on the system version you may be able to use songs from Apple Music, but will not be able to export the finished slideshow with the Apple Music sound track.

It helps to open the Music.app before you try to add a song to your slideshow and to play the songs you want to use, to force a download from iCloud.

Photos for Mac will bestowing the songs you can use highlighted, while the songs you cannot use will be shown dimmed.

An example from my Mac: Two songs have already been downloaded from iCloud and could be selected:

Re: Why can’t I access my music library from my photos slideshow

Wondering if you are using a computer, iPad, iPhone, or other device,

and

which operating system is installed

and

whether you are using Apple's Photos app to create the slideshow.

Discussion thread with tips on adding music

Create Photos Slideshow w/Playlist - Apple Community

Also: Apple support has an article online that may be helpful:

Play videos and slideshows in Photos on iPhone - Apple Support

eg: If you scroll down to: "Make and play a slideshow"

says at the bottom:

"To change the slideshow theme, music, and more, tap the screen while the slideshow plays, then tap Options.

Note: You can also make a slideshow from an album. Tap Albums, tap the album you want to create a slideshow from, then follow the steps above."

All the best :-)