Sorry that you cannot add photos in Lightroom Desktop as the Add Photos button is not working correctly.
Which version of Lightroom Desktop are you using and what is your computer OS?
Would you mind signing out of the Lightroom Desktop app and let us know if that helps?
If that does not help, please try resetting the Lightroom app preferences to default? Press and hold the Shift + Option + Delete keys(Mac) or Press and hold the Shift + Alt keys while starting Lightroom. A dialog box should appear asking you restore the preferences.