Pioneer DJ dropped support for Windows 8.1 and older in version 6.6.0. You must be on Windows 10 (Build 1909+) or Windows 11.
Support for this popular digital music service was added, allowing professional DJs to play tracks from its massive catalog directly within rekordbox, both online and offline. Improved Streaming Workflow: pioneer dj rekordbox 6 profesyonel v664 win