Hi,
When using vtk.js from unpkg.com with a script tag, you can only do var actor = vtk.Rendering.Core.vtkActor.newInstance();. The import approach will only work in a build environment as outlined in the docs: https://kitware.github.io/vtk-js/docs/intro_vtk_as_es6_dependency.html
Let us know if you need any clarifications.