don't have a toolbar in my photos app?

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)

Next log into another user account and see if the problem persists there in Photos. (This tells us if the problem is limited to your user account or is system wide).

If both show the problem boot into Safe Mode (Use Safe Mode to isoloate issues with your Mac and Playing Safe: what does Safe Mode do?) and try from there. Reboot normally and test again.

NOTE: Safe Mode boot can take up to 10 minutes as it's doing some system cache cleaning, volume verifying and directory repairing. 

But if you are really still running OS X 10.11, you need to use the arrows to the right of the traffic light to step between Moments, Collections, Years, all Photos. See the Help page: https://help.apple.com/photos/mac/1.2/?lang=en#/pht56eafa987

Are you in Full Screen Mode? Then the toolbar may be hidden, if the option "View > Always show Toolbar and Sidebar in Full Screen Mode" is not enabled.

Additionally: f you are not in FullScreen Mode, you will be seeing only "Moments", if the window is too small. Try to drag one of the corners of the Photos Window, to make it wider.

If the window is too small, you will be seeing disclosure arrows to the right of "Moments ^v"

What version of Photos do you have? "Years, Months, Days" only exists in Photos 5.0 (part of macOS Catalina). "Moments" was in earlier versions.

Hi léonie, sorry to bump this thread. But, I'm curious if you have any insight into this issue: https://discussions.apple.com/thread/251121054

Thank you!