What changed, before you no longer could update iCloud Photos?
Is iCloud Photos enabled in the System Preferences > iCloud > Photos > Options? If not, enable this as well.
You can only turn on iCloud Photos in the Photos > Preferences > iCloud, if the library is the System Photos Library. You enable this in Photos > Preferences > General: Use as System Photo Library. This option should be grayed out.
Where is your Photos Library stored? On macOS 10.14 Mojave the library needs to be either on the system volume or on an external volume with the file system format MacOs Extended(Journaled). The volume needs to be locally mounted, not a NAS, and must not be used for Time Machine backups.