photos with no side bar

Enter in user library see this article macOS Sierra: Library folder

Find a folder preferences > drag and drop com.apple.system.preferences.plist to the desktop .

There might be another plist com.apple.sidebar.plist in your Mac as for every Mac the plist might vary , drag and drop this also to the desktop , restart the machine and then open photo application , try this method otherwise switch to a new user account see this article How to test an issue in another user account on your Mac - Apple Support

If the window is too small, it won't display the sidebar. Otherwise, I don't know why you would not have a sidebar.