Apple - Can't add items to iTunes library
It's a known bug since the release of macOS 10.14.5.
The blank window that shows up when you try to add an item to your library is actually supposed to contain new terms of service that you have to agree to, but it's bugged on macOS and just displays an empty page instead.
The only solution right now seems to be to go to Apple Music on any of your iOS devices (iPhone, iPad) and add a random item to your library there, then restart your iTunes on Mac and it should be fixed.
This bug has been around since 10.14.5 Beta and Apple still hasn't fixed it, which blows my mind, because it basically makes Apple Music unusable for anyone without an iOS device.