The ambient light sensor has to be locked before measuring. Please make sure that the arrow icon on the sensor head points to the closed lock icon and not to the open lock icon.
The picture shows the ambient light sensor of X-Rite i1 Pro3 in the locked position.
If you use a measurement table, please make sure that the correct measurement device is connected. The measurement device X-Rite i1 Pro 3 can only be used with the measurement table X-Rite i1 iO Pro, the measurement device X-Rite i1 Plus 3 can only be used with the measurement table X-Rite i1 iO Plus.