From:
Apple Watch faces and their features - Apple Support
"The Portraits watch face uses Portrait mode photos from your iPhone to create a multilayered watch face with depth. You can choose from three different type styles and select up to 24 photos. A new photo appears each time you raise your wrist or tap the display.
The Portraits watch face is available on Apple Watch SE and Apple Watch Series 4 and later and requires Portrait mode photos taken on an iPhone with iOS 10.1 or later."
The first (and simplest) thing to try if something isn’t working right is to restart your Apple Watch and its paired iPhone. If necessary (device not responding), try a forced restart. Both methods can be found here:
How to restart your Apple Watch - Apple Support
Restart your iPhone - Apple Support
If that doesn't help, the next steps would be to unpair and pair your watch again and restore from the backup just created. This should result in no loss of data. It deletes all temp and corrupted files and re-indexes the file system.
Unpair and erase your Apple Watch - Apple Support
Set up your Apple Watch - Apple Support
