Shared photo album - There was an error adding the selected items to...

The Shared albums are stored in iCloud, but are not using your iCloud Storage subscription. The shared albums are stored in a different part of iCloud, that is free, but limited. iCloud will prevent us from adding more photos to a shared album, if we have exceeded one of the limits. Then we may have to wait, until we can add more items, or even have to delete previous ly shared items, see: iCloud: My Photo Stream and iCloud Photo Sharing limits

But what is the format of the photo or video you are trying to share? Are they in a supported format?

See: How to use Shared Albums in Photos on your iPhone, iPad and Mac – Apple Support (UK)

Shared Albums support these image formats:

• HEIF
• JPEG
• RAW*
• PNG
• GIF
• TIFF

One more thing - do you only get the error message when trying to share a photo, or also, when you try to add adjustments to the image or when you are trying to export the photo?

As a test - to check, if there is a more problem with the photos - select a photo, then use the command "File > Export > export unmodified original". Can Photos access the original and export it?