What do you mean with prerequisite? My vessel has everything equipped to fulfill the mission... Just to be sure: 7,000,000 km means an orbit somewhere between Eve and Moho right? Equals Sun Periapsis of 7,000,000,000 m on the map... 7000M in the top bar where it shows the distance to surface... 7 Gm in MechJeb's Orbit Info and Surface Info windows... Edit 1: I just noticed that I got the same problem with the satellite challenge where you have to put a satellite in a specific orbit around Kerbin. Edit 2: I had a look at the code of the mission. Is it possible that not only my periapsis but also my apoapsis has to be below 7,000,000 km? Theoretically it would be enough if my periapsis is below 7,000,000 km and I am orbiting for 2 days there. But as it seems the code is checking if my apoapsis is below 7,000,000 km which is not absolutely necessary... Can you remember how you succeeded the mission? Solution: I figuered out that both your periapsis and apoapsis have to be below 7,000,000 km (same applies to the satellite challenge). In my opinion there should be either a change in the mission description or in the implementation of the objective validation, so it's actually sufficient to orbit below 7,000,000 km for 2 days no matter where the apoapsis is.
