When a world uses the "Packets" noble payment system, there are 3 requirements that must be met before you can create a nobleman.
1. You need enough farm space - noblemen require 100 Population space in order to be built.
2. You need enough packets. Packets can be purchased with resources and stored in the Academy.
3. You need enough 'academy levels'. The number of noblemen you can recruit is restricted by the number of academies you have. For every academy level you can have 1 nobleman or conquered village. Academies on Worlds with the Packet System can have a maximum of 3 levels.