The mission is in the pending section, which means it hasn't gone through the OFPEC review progress. This makes it very hard for me to say what's wrong.
As the message tells you, you're missing an addon, so maybe the addon you're missing isn't listed in the mission info.
Your best shot might be to try contacting the author directly, or by posting a comment on the mission, where the author infact is asking
"Is anyone having any problems with playing this Mission at the moment?" 
Topic: Operation Redknight (Read 954 times)