Global Entry: could not schedule interview within next 30 days

You should schedule an interview. From the CBP web site (typographical errors theirs):

You must schedule an appointment within 30-days of receiving you conditional approval, but the actual interview date may be outside that window. Not scheduling the interview will result in cancelation of the application and no refunds will be granted. Also note that canceling an appointment that has reached the 30-day period from the date of conditional approval can cause your application to be cancelled. You may reschedule as often as needed outside of the 30 day window.

To answer the rewritten question, it isn't uncommon for really busy airport offices to show no available appointments at all; the office at the airport in Toronto is like that too. All you can do is schedule an appointment somewhere that does have them available before the 30 days is up, then regularly check the office where you want to do the interview looking for cancellations or new dates to open up (you are always free to reschedule an existing appointment). If you never get a date at Charlotte and the appointment you do have is approaching, you'll need to choose whether you want to travel to the appointment you already have or reschedule it for a later date in the hope that some time at your preferred airport will eventually open up.