Room Sensors and Tank sensors have a physical safety switch that prevents the unit from operating when it is not assembled correctly.
The safety warning can appear at installation, or when the battery's are replaced and the front cover is not placed on its back plate correctly.
This can also occur when there is a faulty installation where Room sensors are not securely screwed to the wall ( which can also lead to detached battery issues as well)
Please ensure the front of the unit it fitted flush all the way round with the back plate, and the two retaining screws are tightened up.
When assembled correctly the message may take a few minutes to disappear then the sensor next communicates in.