If you’re trying to communicate and there’s no actual controller on duty at the airport you’ll be using the Unicom system. It shows up as no frequency info available.
Edit: Depending on the server you’re using (which is good information to know) ATC may not be available. Here’s the basics: Casual server has no operating ATC but unicom is available. TS1 and expert utilize unicom and live controllers if they’re active.
Just in case, here’s some background on unicom:
Apologies if any of this is redundant or if you’re already familiar with all of this.