When connecting the CDJ-2000NXS2 via HID in Serato DJ 1.9.5 or above, it's possible that you may see the following error message when using macOS 10.12 Sierra;

This occurs when the macOS user does not have administrator privileges for the signed in user account.

The default user type when setting up a Mac for the first time, defaults to Admin, but you can check this via System Preferences --> Users & Groups, and ensuring the 'Current User' indicates 'Admin' below it.