Why can't I (share) email photos from PHOTOS? MAIL is not an option to send/share

You Mail/Accounts preference pane should have your email accounts listed:

If you have that in Mail and are not getting mail in the Share menu in Photos

do the following: boot into Safe Mode (How to use safe mode on your Mac - Apple Support) by booting with the Shift key held down and check there to see if the problem persists.  Reboot normally and test again.

NOTE: Safe Mode boot can take up to 10 minutes as it's doing the following; 

• Verifies your startup disk and attempts to repair directory issues, if needed

• Loads only required kernel extensions (prevents 3rd party kernel/extensions from loading)

• Prevents Startup Items and Login Items from opening automatically

• Disables user-installed fonts 

• Deletes font caches, kernel cache, and other system cache files

You must be using the Mail app as your email client for whatever email provider you use before Mail will show up in the Share menu.

• Launch Mail.app.
• Open Mail > Preferences > General
• Enable "Default email Reader: Mail.app".

Then you should be seeing Mail as a sharing option in Photos for Mac.

The sharing option may vanish, if you select too many photos at once to share.

Do it the other way around.

Create an email. Click the add image icon. (top right of the email window)

Select the image from photos