On my OPS there was only one field available but I set to build lunar base to get more free fields...
As I started construction of lunar base I notice message in Construction panel that stated that
there are -2 fields free?! ![OPS fields error OPS fields error](/engine/classes/js/tinymce/plugins/emoticons/img/belay.gif)
I click on Overview to see is this correct and I saw
THIS?! ![OPS fields error OPS fields error](/engine/classes/js/tinymce/plugins/emoticons/img/no.gif)
As I know you can not build something if you don't have free fields...