You are being prompted to enter a registration code as the 767-200/300 installer has detected that you do not have FSUIPC installed on your PC.
What is FSUIPC?
FSUIPC is a 3rd party program that is used by various gauges in this aircraft to communicate with the inner working of Flight Simulator.
Why am I being asked to register?
You are being asked to enter your registration code in case you want to unlock the full version. The version of FSUIPC that is included is licensed for this title so no serial number is needed and you can simply cancel the registration screen and your installation will complete as normal.
You only need to purchase a registration if you wish to use the FSUIPC interface yourself.