Hi there keithfromsmyrna,
To confirm, when trying to add music to your slide show you are following the step below, correct?
7. To select music for the slide show, click the Music button
, click the down arrow next to Music Library, then select a song or songs.
Choose Theme Songs from the pop-up menu to see music included with Photos, or choose Music to see music from your Music library. Drag the songs you selected to change the order they play in. To delete a song, select it and press Delete.
Note: If you don’t see music from your Music library listed, close Photos, then open Music and sign in if you haven’t already. With Music open, open Photos again, then select your slide show. Click the Music button
and choose Music from the pop-up menu to see your music list appear.
That information is step 7 under the "Create a slide show project" section from this article: Create slideshows in Photos on Mac It has other helpful information on creating a slide show.
If you have been following those steps, does this appear to happen with any slide show you try to create in the Photos app? We ask this because it can show if the issue is with any slide show you try to create or a specific one.
If it's happening with multiple slide shows, you can see if restarting your MacBook Air may help. This can refresh the software and help resolve unexpected issues. The steps below show how:
Restart your Mac
* On your Mac, choose Apple menu
> Restart.
* If you don’t want app windows that are open to reopen when your Mac restarts, deselect “Reopen windows when logging back in.”
Those steps can be found in this article: Shut down or restart your Mac
Please let us know what you find or if you've already tried those steps.
Take care!