I just started flightgear after about a week or two and was annoyed that I couldn't get past the selection screen (where you select aircraft, location etc). The Fly! under the plane icon at the bottom was slightly more grey then it was white.
I thought maybe because I had an older version it wouldn't allow me to start (2018.1.1) but after installing 2018.2.2 it was the same.
It took me some time tinkering around and going through screens when I saw that the plane I had selected had an "Update" button next to it. After I pressed that, the Fly! went white.
IMHO the disabling of the Fly! button should not occur without proper feedback what the problem is. Apparently it was easy: update the plane I wanted to fly and go. Why I had to update the plane before I could fly is a bit unclear to me. At least when you click on it there should be a message why it is disabled.
Also, the greying out of Fly! is a bit too subtle. It first it didn't even register and I was just clicking on it and wondering why it didn't work.