When should I use indefinite article ahead of "certain"

The system determines at what altitude can a plane fly in a certain
airspace

or

The system determines at what altitude can a plane fly in certain
airspace

Word complains when using "a certain". Yes, I know I should be guiding myself after Word, but it often led me to insightful answers. In this case, I'm curious not only why it complains but also why to me the 1st version sounds better. I'm not a native speaker :)