What version of ANSYS are you using?

Student or other type of license?

What units are the data in the STL file?  Inches, mm?

The way to use this file is as a template to create real solids from scratch while using the STL facets to help size the features.

Here is the facet body...Create a Plane then use the Tools > Extract Curves tool to get this...

 You can sketch over those rough shapes to make surfaces that can be pulled into solids and then worked on from another angle.