Uploading photos to shared album

hi, if I go to Add photos in that album, I can add photos. But, if I select a person in people's album, and chose to add that person in the shared album (named after that person), I get that error. Basically, I have created a shared album of my son called, Samuel, and I want to put all of Samuel's photos in the shared album. It's not working.

Hi, yes, I can open these images in edit mode, and yes, I use iCloud photos with optimise Mac storage selected, thanks