title: add-apt: command not found even after installing software-properties-common
body: <p>Even after doing in Debian 8 this command :</p>
  <blockquote>
  <p>apt-get install software-properties-common</p>
  </blockquote>
  <p><a href="https://i.stack.imgur.com/mp71P.png" rel="nofollow noreferrer">add-apt still can't be used</a></p>
  <p>Thanks by advance for your help</p>