How do I create a slideshow from an album on Photos?

Is your album sorted in a specific way, and you want to use the same sorting in the slideshow?

Then do not use "File > Create", but select the slideshow in the sidebar of the Photos window and ctrl-click the album in the sidebar. Use "Create > Slideshow" from this menu. Otherwise the sequence of photos in the resulting slideshow may be differnet from the sequence of photos in the album.

select the photos and under the file menu ==> create ==> slideshow create the slide show

Create and work with slideshows in Photos on Mac - Apple Support

LN