The error message here is out of date --
simtk.unit was renamed to
openmm.unit by the OpenMM maintainers. So this really means that you don't have OpenMM installed on your system. (Error message will be fixed in this pull request).
If you're not planning to use OpenMM for dynamics, then you probably want to load a trajectory using something like the solution in this answer.
If you are planning to use OpenMM for dynamics (as the examples do), then you'll definitely want to install OpenMM! You can do that with
conda install -c conda-forge openmm. Note that OpenMM can not (yet) be installed with
pip. You can see other optional installations that are supported by OPS in the docs on integration with other packages.