the error is caused by residual pressure at power on or the sensor being out of calibration. Open the drain valve and restart the instrument. If the error persists, it is likely that the pressure sensor is out of calibration.
There is an adjustment screw on the rear panel to zero the 'true' reading. If the value is out of calibration the error message will be displayed. Press clear and the instrument will continue initializing. Press shift and then 6 and you can see the true value, carefully adjust the rear panel using a small screwdriver until the value reaches zero. Power off and on.
If the error persists it is likely the pressure sensor has failed and needs to be replaced.