Check if the PC you are using fulfills the minimum system requirements for rekordbox dj. Alternatively, lower the output quality settings in the environment settings.

Check the version of rekordbox.

See [Help] > [About rekordbox] at the menu bar.

If "x64" is not at the end of the version name, please download rekordbox Windows for DDJ-RZX from here, not from rekordbox update manager.