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I do not use WordPress much, but, basically it is saying that your server does not have a MySQLi database installed. Are you using your own server or paying for a hosting service? If your own, is it a local test server or a full Wamp or Xamp system? If it is a paid server, did you log into the control panel of the server and make sure you set up a database? On PHP, either local or hosted, when you install PHP, it comes with certain options. Usually, MySQLi and Json are both defaults. We need more info to help you further!
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