system photo library unavailable

You need first to make your current Photos Library your System Photos Library ( System Photo Library overview).

• Click the "General" tab in the Photos Preferences, then click "Use as System Photos Library".
• Then click again the iCloud tab in the Photos Preferences and try again to make the library your iCloud Photos Library by enabling iCloud Photos (From iCloud Photos).

See the thread https://discussions.apple.com/thread/8536957