I successfully set up PHPFarm and Apache following some slightly different instructions.
I'm no Apache configuration expert, but one thing which is in those instructions but not in the link originally posted by the questioner is declaring the PHP instances as FastCGI applications. Both tutorials talk about a config file, but the one I used has a config file that looks like this:
#used for multiple php versions
ScriptAlias /cgi-bin-php/ /var/www/cgi-bin/
FastCgiServer lines are not found in the tutorial referenced in the original post.
Furthermore, I tried to add a new PHP version to one of my Apache sites today, and when I forgot to update this config file to include a reference to it as a
FastCgiServer, I saw precisely the error mentioned in the question. So it looks like this could well be your problem.
(Note to anyone who finds this via Google - if you're adding a new PHP version to an existing PHPFarm setup like I was, you'll also need to add a new script in
/var/www/cgi-bin, but that's not the original questioner's problem.)