Is it a new product? If yes, correctly insert the battery and/or check the connection between the 9 V contact or the metal strips.
Some of our bathroom scales are supplied with lithium batteries (one or two depending on the model). The batteries must be inserted into the battery housing located underneath the scales. Always make sure that the + sign is visible after inserting the batteries.
Other models use alkaline batteries (9 V) (not included). The battery must be inserted into the battery housing. See the instruction manual for further details.