Title is pretty self-explanatory here. AGOS schedules a stage lock, but the staging never actually gets locked. This was also an issue in the 1.x.x version of AGOS which caused the feature eventually be dropped.
Error message: [WRN 13:12:42.717] [Part]: PartModule AGOSModule at reserveBatteryPack, index 0: index exceeds module count as defined in cfg. Looking for AGOSModule in other indices... [ERR 13:12:42.717] ...no AGOSModule module found on part definition. Skipping...
Temporary workaround: Switching to a vessel from the tracking station or space centre view and then returning back to the space centre will unlock the button for use again.