I had the same this morning, but I don’t think it’s related to the Washington Landmarks scenery as I don’t have it, but had the problem nonetheless.
But listen to this, because the plot thickens.
This morning i did a 3 hrs session with MSFS to check out the new 40th anniversary features.
One of the first activities I tried was the Chicago tour from Meigs and I too found out that the yellow “Fly” button was greyed out and I couldn’t try the mission.
I then checked some other missions, saw that they worked, did a couple of them, and then went back to the Meigs one. Lo and behold the yellow “Fly” button was active this time 
! I then launched the mission, did a quick flight around Chicago up to Wrigley Field and landed back at Meigs.
I then flew a few other missions and went back to the Meigs one, as my first tour had been a short one and I wanted to try it again. And this time around once again the “Fly” button was greyed out and I couldn’t start the mission.
Mind you, this all happened in the same MSFS session. I didn’t quit and restart, didn’t mess with my community folder, I didn’t lose connection to the servers, nothing! I may have changed some controller options but other than that I didn’t do anything at all that could explain the magical appearance and disappearance of the ability to press the “Fly” button.
And every time I did wait a bit for the “Fly” button to turn yellow, as sometimes it takes a little bit after you load an activity for it to become active. But nothing.
I’m puzzled 
.