Do you have enough Capital Ship Crews?
Correct me if I'm wrong but that could also interfere I believe.
Nope, it will actually let you have negative capitalship slots if you spawn them via abilities lol.
My code and yours are identical. Is there something else I am missing?
The code looks fine, again the only thing I've found that would cause that is not having 150 fleet supply available or you are not playing as Vasari Rebels. Besides that I have no idea what could be wrong.