Hi @KSartakov! Welcome to the forum!
There are a couple of reasons you might be seeing this error. You’ll want to make sure you’ve built the OpenMC library and executable with DAGMC enabled (instructions here).
Also, if you built OpenMC once without DAGMC, installed the Python API, and then re-built OpenMC with DAGMC support, do make sure you install the Python API again so the updated OpenMC library gets included with the Python package. The plotter will rely on this library rather than the one placed in the installation location by CMake.
I hope this helps! Let us know if you have any additional questions.
-Patrick