I recently bought my first iMac and PSE2020 to go with it having used PSE on PCs very happily since PSE3. When trying to open images (RAW & jpeg) from the Photos Library (containly thousands of photos) I get the error message "Could not complete the request because Photoshop Elements does not recognise this type of file".
It may be relevant that the Photos library disappeared a short while after download but was recovered.
Also I did manage to access a couple of RAW files before this problem occurred, but instead of opening as .CR2 files as in the past, they opened as fullsizeoutput_24e0.jpeg. I am completely mystified.