install or enable php's mbstring extension code example
Example 1: install php-mbstring in ubuntu
sudo apt-get install php7.2-mbstring
sudo apt-get install libapache2-mod-php7.2
sudo service apache2 restart
Example 2: Install or enable PHP's gd extension
apt-get install php-gd
Example 3: Install or enable PHP's xml extension
sudo apt-get install php-xml