Sorties say you need to unlock an already unlocked node, preventing you from entering.

[(PSN)Madurai-Prime]

My entire starchart is unlocked, clicking on a sortie says "You need to unlock Valac (Europa) to play this mission."

I go to the Valac, Europa node, and the sortie isn't available, just the regular node. 

I can enter the sortie if I enter regular Valac and abort mission immediately after, though. That obviously gives me a failed mission though.