Citation Latitude First Officer reported pulling aircraft chocks causing the aircraft to roll 20 feet on the ramp before the Captain could stop it. First Officer reported failing to set the parking brake while performing the Captain's pre-flight duties who had sustained an injury earlier in day.

Narrative

The flight was scheduled as a reposition from ZZZZ to ZZZ. Upon arriving at the airport; the captain sustained an injury when exiting the van; but determined they felt they could continue. Durning preflight; the first officer missed setting the parking park. Due to the captains injury the first officer pulled chocks while the captain monitored the cockpit. As a result of the missed parking brake when the first officer pulled the chocks the aircraft began to roll due to the uneven surface of the ramp. The first officer was already out from under the wing and was unable to rechock the airplane when it began to move. The plane rolled approximately 20ft before the captain was able to set the brakes. No people; aircraft; or structure were hit or within 10ft of the aircraft when it came to a stop. The plane was towed back to its original parking position. At this time the Captain determined that his injure was too great to continue and they discontinued the flight. Suggestions: Before removing the chocks both pilots must verify that the parking break is set.