Photos can not be processed or edited after they have been moved to a server, another computer or opened by a different user. They are marked as Read-Only with a crossed-out pencil icon at the bottom of the thumbnail.
When such an issue occurs, it means that a user does not have the read/write permission rights for the file or folder. Navigate to the Get Info option by right-clicking on an image to see if this is the case.
To prevent the issues related to the processing and editing, enable the Make new files writable by everyone option by navigating to Preferences -> Image.
For existing files, find the appropriate folder with those files, highlight it, and press Command+I or right-click and choose Get Info.
At the bottom of the Info window, in the Sharing & Permissions section, set Everyone to Read & Write. You may need to unlock the pane via the padlock icon at the bottom right (which requires your OS username and password).
To apply the changes in permissions to all items in the folder, click the settings icon (next to the "+-") and select Apply to enclosed items...