is this to your phone or computer try turning the blue tooth off then turn it on &try again. Another thing to try is see if it will setup to a different phone or see if a friends fitbit will setup to your phone. The other thing is it could also be broken
Most keyboards work the same way. Make sure your tablet or phone has BT turned on, and then enable scan more. On the device, hit the bluetooth pairing button. Often a keyboard will send a code to the device that recommends a key sequence needed to complete pairing. Here is a video from the same manufacturer that probably works the same way. PureGear Folio 10 inch
since you didn't mention what the 'box' is, (make and model), either contact the place you bought it or go to the manufacturer for help in figuring it out.