Pictures can't be imported from Photos. Why?
This issue is related to iCloud. CamScanner cannot import images which have been synced to iCloud and cleared from your iPhone. Normally, iCloud will clear the original images from your phone and only keep the preview ones to save your phone storage.
To solve the problem, please open those pictures in your iPhone Photos to download them from iCloud. After that, you can import them into CamScanner.