Photos: Can't edit.

As a test launch Photos with the Option key held down and create a new, test library.  Import some photos and test to see if the same problem persists. Does it? This tells us if the problem is limited to your current library or is more wide spread.

Then open your original library and test again.

If it still can't edit create and log into a new, basic, admin user account, Set up users, guests, and groups on Mac,  repeat the above test library and see if the problem persists. This tells us if the problem is limited to your user account or is system wide.

Note:  do not launch any 3rd party apps in the account so as to not install any of the supporting files that most create.

If the problem isn't present then your original account has software installed that is causing the problem.  

Sometimes something happens to cause the Photos database to lose the connections between its editing files and its picture files. This can happen if you use disk "cleaning" software which doesn't take proper care of the Photos Library, and that can corrupt the database and cause all kinds of havoc. I've also seen it happen for no reason that I can figure.

If you haven't re-started your computer, you should try that. Sometimes that's all it takes. If that doesn't do it, try exporting the picture to your desktop using File>Export>Export Unmodified Original. If that works, you can re-import the picture and it will work fine. Of course, you'll have two copies of the picture, and you may have to copy and paste keywords to the new version and delete t he corrupted one.

If you can't make that work, try rebuilding your Library. Close Photos and re-launch by "option-command-clicking" the app icon, and telling the pop-up dialog to repair. It may take a few minutes. As always, it's good to have a backup of the Library before doing something like this. If you use iCloud to synchronize Photos on your devices, this may confuse it for awhile. I would disconnect the Library from iCloud before messing around with it.

It's always worthwhile to re-start the computer in Safe Mode. This bypasses certain potentially disruptive processes. Safe Mode is different for different computers, so see this: 

           Start up your Mac in safe mode - Apple Support 

The idea is to find out if the problem is with the Library, with your account, or with caches and login items, or even one or two pictures that are corrupted-- stuff like that. One of these steps may fix the problem, but their failure to work is also diagnostic.

Let us know...