There may be following possible causes for the same:
Ignition Wire not connected: Make sure trackers ignition wire is connected to vehicle ignition wire. Till then, to generate reports in this case , uncheck 'Ignition Wire' option in 'Tracker Management' from admin account and then generate report, they will generate fine. After connection issue is rectified , enable ignition sensor.
Ignition Wire Connected still no data in reports : To generate reports in this case , uncheck 'Ignition Wire' option in 'Tracker Management' from admin account or vice-versa and then generate reports, they will generate fine.