I downloaded Graphviz 2.38 MSI version and installed under folder C:\\Python34, then I run pip install Graphviz, everything went well.
When solving this issue for myself, I used this GitHub tutorial, which analysed the cause of this issue. If we read in between the lines, it says it needs system as well as python graph viz. In addition to conda install, we would need to run:
conda install -c conda-forge python-graphviz
Then restart the kernel; it works like a charm.