This can occur if you have a Power Amplifier plugged into a to LabQuest that is connected to a computer running Logger Pro data-collection software.
If you left an entry screen open on LabQuest from either the Power Amplifier or the Audio Function Generator before you connected LabQuest to the computer, it may have caused communication issues between LabQuest and Logger Pro.
To resolve the issue, disconnect LabQuest from the computer, navigate to the entry screen that was left open and close it. Reconnect LabQuest to the computer. In Logger Pro, select Setup Sensors from the Experiment menu and open the LabQuest interface.
The Power Amplifier should be recognized as a sensor and you should now be able to access control of the frequency with the Power Amplifier button.