I don’t know how to activate or deactivate my computer for use with my rekordbox ver. 6 subscription plan.

You can activate/deactivate your computer on My Page in rekordbox.

Activate:

Click [MY PAGE] and turn on the [ON/OFF] button to the right of [Activate this computer] on the [PLAN] tab.

Deactivate:

Click [MY PAGE] and turn off the [ON/OFF] button to the right of [Activate this computer] on the [PLAN] tab.