This issue is generally caused by missing files in your piwik directory. To fix this issue, please reupload all files from the latest Matomo (Piwik) version to your server.
If after re-uploading all the files from the latest version you still experience this issue, try to restart the web server.
If you still have the issue after doing those two steps, it may be a bug in Matomo (Piwik). Please create a bug report.