It just means that they've not released any additional slots there yet. That happened to me at ORD a couple of times when I was trying to schedule mine. Some more slots opened up a few days later. Just keep checking periodically. I checked 2-3 times a day

If you're planning on traveling somewhere with an enrollment center, try that. Or, if you have some spare time, try for a drop-in appointment. There was a young lady dropping in when I had mine scheduled. Another CBP officer happened to be available at the time, so they took us both right then. YMMV, of course, but doesn't hurt to check it out if you have the time.