I created a source folder that had VTK 9.1.0 in it and then I created a build folder. When I run CMake it works until it hits the CMakelist error I listed above.
The source I downloaded here worked just fine. Do you have specific commands I could run to reproduce the behavior (including downloading and extracting the source)?
Hello! I ended up reinstalling VTK and the problem resolved itself. I believe that during installation, it must not have downloaded all of the files. Thank you for your help!