Learn German: Weißt du, ob wir das Abo ab sofort aktivieren dürfen? - Do you know if we are allowed to activate the subscription starting now?

The preposition ab means "from" or "starting at" when talking about a point in time. The word sofort means "immediately". Together, ab sofort is a very common fixed phrase meaning "starting immediately" or "from now on". It tells us exactly when the permission to activate the subscription begins!

Why are there two verbs at the very end of the sentence (aktivieren dürfen)?

The word ob (if/whether) introduces a subordinate clause, which always pushes the conjugated verb to the very end. Here, the conjugated verb is the modal verb dürfen (are allowed to). It gets kicked past the infinitive verb aktivieren (to activate). In a normal statement, the order would be: Wir dürfen das Abo ab sofort aktivieren.

Can I use wenn instead of ob since they both translate to "if"?

No, you must use ob here. In German, ob specifically means "whether" and is used for yes/no questions or uncertainties (like asking whether or not we are allowed to do something). wenn is used for conditional statements ("if it rains, then...") or for time ("whenever").

Is das Abo an abbreviation?