Call to undefined function twig_lower_filter()

[mattab]

This error is due to logic in twig at: https://github.com/fabpot/Twig/blob/master/lib/Twig/Extension/Core.php#L1187

The |lower filter is only added to twig if the PHP mbstring extension is enabled.

mbstring is a non-default extension. This means it is not enabled by default. You must explicitly enable the module with the configure option.
http://www.php.net/manual/en/mbstring.installation.php

The fix for this bug is to enable mbstring.

[mattab]

In 7b739c2: Fixes #4999 Add mbstring to the list of "required extension" in the system check

[karussell]

I got the same error after upgrading from ubuntu 14.4 to 16.4 and php5 to php7 and just as a reference for others can be easily fixed via:

sudo apt-get install php7.0-mbstring