
Basically to use your DDJ-SZ with rekordbox 6, you either have a paid subscription activated (ie. Core Plan or higher plan) or you would need to have one of the hardware unlock products also connected (ie. CDJ-3000 or DJM-V10).
You always have access to Performance Mode on rekordbox 6, but some functions and features of Performance Mode are only unlocked by a paid plan or hardware unlock device.