My car has been in the workshop for a month. When I got my car afterwards, tried to add the fuel-up on the 29th Aug. but the distance is not appear in the distance (km) column.
The distance is shown as the 303 Km. It looks like you have every fuelup entered as a partial fuelup. Partial fuelups will not calculate fuel efficiency.
The partial fuelup should be checked only if you do not "top off" your tank. If your tank is completely full, regardless of how much fuel is added, it is not considered a partial fuelup.