This is the moment of validating every parameter that should be inserted on the evaluation. Thus, it is pretty useful to understand the main warnings on the surfaces importation. Usually, when you import an invalid surface the box would be in red, and by holding the mouse within its limits you could see the message about it.
The first common issue here is about importing a surface which does not meet the block model limits, which should be related to the origins or the surface size. To check this issue verify your origins and check your coordinates by following the steps mentioned on this page. An additional error is based on the name of the headers of the surface file, which might be always “X, Y, Z”. Thus, if it has any other name or type, correct it accordingly with this statement.
It is always worthwhile to identify the warnings and validate your surfaces using this page if you have any warning about them.