I ended it with a friend through text and told her I'll miss her. She responded "I do not know what to say." What does that mean? Any chance she will text me back to elaborate on that, should I text her or leave it as is?

Seriously. It seems you have no idea how to treat people. TALK TO SOMEONE IN PERSON - FACE TO FACE - ESPECIALLY WHEN YOU WANT TO "END" THINGS, whatever that means.

Have the courtesy to make a decision and talk to someone about what you have decided. It is hard to be misunderstood that way and the other person then has the chance to ask a question or make a statement back to you.