This error means that the PHP of your server doesn’t have the mbstring extension, which the Full Content add-on needs to convert the encoding of the texts it gets from other websites.

Fatal error: Call to undefined function mb_convert_encoding()

WPeMatico shows it for you.

Go to WPeMatico → Tools, open the System tab and look for php.net/mbstring in the PHP Environment table. If it doesn’t show a check mark, the extension is missing.

Depending on your hosting.

• Ask your hosting provider. It’s a standard extension and most providers can enable it in a few minutes.
• From your control panel. In cPanel and other panels, the PHP settings page (for example Select PHP Version → Extensions) usually lets you check mbstring.
• On your own server, follow the mbstring installation guide of PHP and restart the web server.

After enabling it, reload WPeMatico → Tools → System to confirm that mbstring shows a check mark, and run the campaign again.