It appears the composer tool is not yet installed. You can install Composer in a few easy steps:
In the piwik directory, run in the command line the following (eg. via ssh):
curl -sS https://getcomposer.org/installer | php php composer.phar installThis 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.