That's not a mistake! Don't worry; the text is probably saved. You need to put a "Sign A" some where the screen. It's in the "Systems" bank. If you want to be fancy, put a "Sign A" where you want the text to be, and "Sign Area A" around the location you want it to show up. IE; I want people to see the text above this rock; so put the "Sign A" above the rock, and the "Sign Area A" around the rock. Experiment, you'll get it. That's fancy, though, so for right now, just start off getting a "Sign A" on the screen. When the character passes over it, the text you wanted should appear, all things having gone right.
It's Bank 0 Object 17 for the "Sign A". It's also clearly marked in writing. Hope this helps!