Now you know ... if the subject is not within a few yards, light from the flash will not reach it. Don't worry about it it's part of the learning process ... you see thousands of people in the stands make the same mistake during the kickoff of every Super Bowl
You have a great vantage point which is a big help to getting good shots. This shot is underexposed, seriously underexposed. Perhaps you were on auto and the meter read the sky. I believe you are looking east here ... try to return to this very spot at various times of the day ... early evening / sunset looks like a great time to be shooting from this spot. Vary your exposures so when you start looking at your pics, the proper exposure will be among them. You will start to get a feel for getting a proper exposure. Use a tripod which will also come in handy getting dandy night shots from this spot !
Keep shooting !