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 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.

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