title: Cannot get IRkernel::installspec() to work, jupyter-client not visible
body: <p>I am still in progress to get Jupyter-Notebook and R working together on Windows 10 machine. After solving the <a href="https://stackoverflow.com/questions/53086530/cannot-install-irkernel-with-install-github-git-not-installed">problem with install_github()</a> I am now facing just another issue. I am still following the official <a href="https://irkernel.github.io/installation/" rel="nofollow noreferrer">installation guide</a>, I am stuck at step 2:</p>
  
  <p>Every time I get this error - I have tried this in R GUI and RStudio, the same:</p>
  
  <pre><code>&gt; IRkernel::installspec()
  Error in IRkernel::installspec() : 
    jupyter-client has to be installed but “jupyter kernelspec --version” exited with code 127.
  In addition: Warning message:
  In system2("jupyter", c("kernelspec", "--version"), FALSE, FALSE) :
    '"jupyter"' not found
  </code></pre>
  
  <p>My attempts so far: It was suggested <a href="https://stackoverflow.com/questions/44056164/jupyter-client-has-to-be-installed-but-jupyter-kernelspec-version-exited-wit">here</a> and <a href="https://datadrivenperspectives.com/installing-r-on-jupyter-725a4643983b" rel="nofollow noreferrer">here</a>, that this could be a problem with the PATH variable setting, so I have added Anaconda to the PATH enviroment. I also installed the jupyter-client via the Anaconda console. No change so far. Any ideas? </p>