Hello, which tool are you using for meshing? Please make sure that your CAD is free from any errors before meshing. If you are using Watertight Workflow Meshing then assuming that you have only one fluid domain, the geometry consists of only one fluid region and no voids is the valid option in the Describe Geometry task.
Similarly, you will have one fluid region in the update regions task which is basically the fluid domain surrounding substracted portion of the CAD. If you are not including more than one fluid region.
To know more about the Describe Geometry and Update Regions task of Ansys Fluent Meshing Watertight Geometry Workflow, please refer to the link of a course given below