Thanks Paperdigits and Stuntflyer,
That ewas very helpful. However now that I am able to find the Preferences, I searched through the most likely places - “General”, “Image Processing”, and “File Browser” and can find no place that I can add the folder that contains everything on my computer to what RT is looking for. As I wrote above, I am able to open photos that are in a file called “Photos” but nothing else that is inside the folder with my name on it. Inside that is my Desktop (where, occasionally, I keep photos I am working on) and also the external “Photos” drive where I keep everything. As a workaround, it is fairly easy to transfer a photo I might want to work on into that strange “Photos” file (which I had never even realized existed before this), but that adds and extra step that I would prefer to skip, if I can. I think RT ought to be able to open files from anywhere on my computer.
Anyway, it seems I am making progress thanks to everyone’s kind assistance.
Photos
Shows the apps that have requested access to your photos. Deselect an app if you want to prevent it from accessing this information.
Note: If you have items that are stored outside the Photos Library, other apps may still have access to them.
… Full Disk Access
Shows apps that can access all files on your computer, including data from other apps (for example, Mail, Messages, Safari and Home), data from Time Machine backups and certain administrative settings for all users on this Mac. Deselect an app if you want to prevent it from having access to your files.
To add an app, click the Add button, select the app in the list, then click Open.
Files and Folders
Shows apps that can access files and folders in different locations on this Mac. Deselect an app if you want to prevent it from accessing files and folders.
… Media & Apple Music
Shows apps that have requested access to Apple Music, your music and video activity, and your media library on this Mac. Deselect an app if you want to prevent it from recording your screen. (Edit: I’m not sure your photos are part of the ‘media library’, maybe that’s only content you have purchased?)
HomeKit
Shows apps that have requested access to your home data on this Mac. Deselect an app if you want to prevent it from recording your screen. (Edit: sorry, I’m not a Mac guy; this seems to be related to home automation, not to your home directory, if there’s such a thing under MacOS.)
Again, your advice is very helpful in moving me toward fixing the problem. It wold be easy to go to the “File and Folders” section of my System and turn RT on if it had requested to be listed there. It seems, though, that RT never asked for permission to be able to access the “Files and Folders”. That is the place in my System where all my other Photoshop and Adobe permissions are kept so if RT had asked to be included, it should have shown up there.
Please run RT directly and not from the file browser from the operating system. Then you should see the preferences. All people mentioned that you should double click in the browser always meant the RT embedded browser.