It appears the composer tool is not yet installed. You can install Composer in a few easy steps:
Download and run Composer-Setup.exe, it will install the latest Composer version and set up your PATH so that you can just call composer from any directory in your command line.
Then run this command in a terminal in the piwik directory:
$ php composer.phar install This will initialize composer for Piwik and download libraries we use in vendor/* directory.
Then reload this page to access your analytics reports.
For more information check out this FAQ: How do I use Piwik from the Git repository?.
Note: if for some reasons you cannot install composer, instead install the latest Piwik release from builds.piwik.org.