Hi Kazabelanka2016,
Thank you for using the Apple Support Communities! I understand you are experiencing an issue using Memories in Photos. I'd like to help out.
Taking a look at this article: View Memories in Photos on iPhone - Apple Support, check out the section titled, 'Create a Memory movie'. Confirm if these are the steps you are taking to create your video.
I would also recommend the following steps:
-Make sure your iOS is updated: Update your iPhone, iPad, or iPod touch - Apple Support.
-Restart your iPhone: Restart your iPhone - Apple Support.
-Test creating multiple Memory movies using different Days or Months.
Keep me posted, if you need continued support.