mb_convert_encoding error: Call to undefined function mb_convert_encoding()

On Windows, uncomment the following line in php.ini, and restart the Apache server:

extension=mbstring

If you still get the error afterwards, make sure that what you see is not cached response.

You need to install the extension. It depends on of your operating system, here are some examples:

sudo apt-get install php-mbstring  # Debian, Ubuntu
sudo yum install php-mbstring  # RedHat, Fedora, CentOS