Read the message in the app. The usual causes are: a required photo is missing, the pre-start check is not finished, the odometer reading is lower than the vehicle's last reading, the vehicle is out with another driver or set to Off the road, or location is off while your rules check where drivers clock in.
If your clock-in location rule is "Only inside the depot", clocking in away from the depot is refused with "You must be at the depot to clock in". Check the driver's home depot and that the depot has a map position (Settings, Depots). Other reasons: the driver is on duty for another company ("Clock out before switching company"), or the account is read-only because the trial ended.