The battery is not properly inserted into the battery compartment or is depleted.
Check whether the battery is correctly inserted in the battery compartment.
If necessary, insert the battery correctly.
If the battery is empty, replace it as follows:
One 3 V battery (CR 2032 button cell) is required.
Depending on the model, open the cover to the battery compartment:
Undo the fixing screw on the battery compartment cover using a crosstip screwdriver and then remove the cover (top figure).
Unlock the cover, e.g., using a flat-head screwdriver (bottom figure).
Replace the battery with a new one. Make sure the polarity is correct.
Reattach the cover.
Depending on the model, tighten the fixing screw.
Dispose of the old battery in a designated recycling bin for batteries.