STL is composed of triangles. It can be messy geometry and may be a problem to mesh directly unless you clean it up in a software that can handle mesh-based geometry such as SpaceClaim. It also doesn't usually have well defined patches (faces), so the default patch based tetrahedral mesher wouldn't work. You could insert a mesh method and set to the patch independent tetrahedral method. Or maybe you got the error simply because you need to set appropriate element sizes. There is an introductory workbench meshing app training course in the Ansys Learning Hub.
It's best not to work with STL when you don't need to. It complicates things. Mesh based data is normally only provided from a laser scan of a real part (reverse engneering). Or it may come from the deformed mesh of a previous analysis in Ansys or other analysis software.
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