This tutorial will explain you how to fix “Fatal error: Call to undefined function mb_substr()” error after template installation.
If you get this error after template installation, this means that “mbstring” PHP extension is not installed or not enabled on your hosting server.
mbstring provides multibyte specific string functions that help you deal with multibyte encodings in PHP. In addition to that, mbstring handles character encoding conversion between the possible encoding pairs. mbstring is designed to handle Unicode-based encodings such as UTF-8 and UCS-2 and many single-byte encodings for convenience (listed below).
To fix this error, you need to contact your hosting provider and ask them to enable/install mbstring extensions for you. Alternatively you can try to install it yourself by following these instructions: http://php.net/manual/en/book.mbstring.php