Hello mollyfromgermantown,
Thanks for posting! As we understand it, you're having some issues with the Memories function in Photos on your iPhone. That's a great feature, and we'll be happy to see how we can help out.
Does this happen with every Memories video you try creating?
You mentioned this is happening when you try customizing the Memories video. Do you mean when you play the video and tap Edit in the top right, or something else?
Does it happen after you change a particular setting in Edit, like Duration?
Do the videos in Memories play as expected until you try customizing?
Depending on the answers to the above and the steps you've tried so far, you might try to Restart your iPhone.
Updating to the latest iOS version could be helpful also: How to back up your iPhone, iPad, and iPod touch and Update your iPhone, iPad, or iPod touch
I hope this gets you started. Feel free to reply back with how it goes.
Have a good day.