The center pad is a square pad in the center of the package, and it is a GND pin with no assigned pin number. It is not a required pin to be connected to GND, so it will work even if it is not connected. However, since this center pad plays a role in heat dissipation, we strongly recommend connecting it to GND for stable operation.
If it is necessary to hand-solder for pre-evaluation, etc., it seems that there will be no problem even if it is not connected.