The hexahedral elements can be used only for structured or semi-structured (i.e., extruded) volumes. In the latter case, the general workflow is that you select the surface to be extruded and the extruded direction. There are small differences depending on whether you work in GiD or ATENA PRE 2024, so kindly specify what pre-processor you are using.

Regarding the error message, if you do not manage to resolve the issue, please kindly send us the model to cervenka@cervenka.cz

Please guide me.

Another question is whether a Unstructured element can be meshed with hexahedra elements? And how can this work be done and how can Unstructured elements be meshed? Please can anyone help me to send my model to him and check it.

best regard.