Void rooms in an ESX sketch are areas that are supposed to be a part of the overall sketch layout, but do not have any 360° images documenting them.
By default our system labels them as void rooms for you to replace, remove or rename later on. See example:
The areas highlighted are areas that are almost impossible to take a 360° image of. However, because these areas are part of the overall sketch, these are taken into account and marked instead as void rooms in the ESX file.