The official website for The Settlers of Catan

Question about General Questions Knights (convertible) - Can I immediately activate a Knight that can be converted into an Action Card if I have acquired him with the help of Bribery?

Answer

No. You may do so one round after his acquisition at the earliest.

Note: The Bribery belongs to the theme set "Politics & Intrigue", which is part of the Catan Card Game – Expansion Set.