In iTunes for Windows, iCloud Music Library isn't turned on by default. To turn on iCloud Music Library:

Open iTunes. From the menu bar at the top of your screen, choose Edit > Preferences.

Go to the General tab and select iCloud Music Library to turn it on. If you don't subscribe to Apple Music or iTunes Match, you won’t see an option to turn on iCloud Music Library.

Click OK.

If you have a large music library, it might take some time to upload and sync your music library across all of your devices.