Unable to use personal music in iPhoto Memories after upgrading to 18.1.1

I created a slideshow of my son’s life and used his own composed music. Having upgraded(!) to 18.1.1, I find I have to make a Memories slideshow (which I’ve done) but can’t use his own music. My son ended his life in April this year, you’ll see how important it is for me to have this slideshow. Please note I’m 70+ and non-techno-savvy.

[Re-Titled by Moderator]