pVACviz is part of the pVACtools package. To install pVACtools, execute the following command on your Terminal:
pip install pvactools
More detailed installation instructions can be found here. Note that the following are the bare minimum you need to run pVACviz. Most users will probably just want to complete the full pvactools install as described here. That includes pVACviz along with all the other components, local installation of IEDB, etc. You can also use the pvactools docker container which contains all tools and their dependencies (including those for pVACviz).
When installing pVACtools for the first time, you will need to manually download the MHCflurry dataset: