Open Photos for your current Photos Library.
Then open the Photos > Preferences > General. Is "Photos > General > Library Location: Use as System Photo Library" selected?
This option should be grayed out; if not, click it. Then quit Photos and restart your Mac.
Open Photos again and let it run, to create previews for the Media Browser.
It may take a very long time for the library to appear in the Screensaver System preferences and to show the albums and moments, when you open it the first time.be prepared to have to wait several minutes.
If the photos do not show up at all: Is your Photos Library in a location, where other applications can see it? In your Pictures folder? Or is it on an external drive? If it is on an external, is the drive formatted correctly as MAcOS Extended(Journaled) and directly connected, or is it a NAS?
Can other applications see your Photos Library? Does the Photos Library appear in Mail, when you create a new message?

If you library is an iCloud Photo Library, the photos need to be downloaded from iCloud to be able to show in the Media Browser.