RStudio v. 1.2.1502-1 does not install the pandoc binary folder. This occurred on both Win7 x64 and on Win10 x64 (in a Parallels VM).
Here is a screen shot
Manually creating the pandoc directory and copying the required file fixes the problem.
Per Kevin Ushey, I needed special pandoc binaries (2.7.1), so to be sure this did not cause the problem, I did an uninstall and a clean install of 1502-1 and verified the installer did not install the pandoc binaries...