P: Enterprise/Education Licenses without Storage are unable to use offline-only mode

A Preference File will survive a Lightroom uninstall/reinstall. Sometimes weird behavior is corrected/cured by resetting the preferences. 

Reset Procedure:

1. Close Lightroom.
2. Hold down [Alt/Opt]+[Shift] while restarting Lightroom. 
3. Overwrite the Preferences when prompted by the dialog.
4. Close Lightroom.
5. Restart Lightroom.

Does the behavior continue after resetting the preferences?

Hello Rikk Flohr, I have exactly the same problem, that the Plus Sign to add photos is not working. I have the latest Version 2.1.1 of LR CC and reinstalled it a couple times. Login and Logout did not help. What to do?

Please update to 2.1.1 via the Creative Cloud Application. There were several bug fixes in this version that may affect your issue. 

Lightroom CC version: 2.0.1

Lightroom does not have a version called CC2019. Can you go to Help>System Info... and get us your installed version number?

Are you selecting your source folder first? It is normal for "Add" to be grayed out if all images in the source folder selected are already part of lightroom.