sudo apt-get install php-mbstring # if your are using php 7.1 sudo apt-get install php7.1-mbstring # if your are using php 7.2 sudo apt-get install php7.2-mbstring
sudo apt-get install php-mbstring # if your are using php 7.1 sudo apt-get install php7.1-mbstring # if your are using php 7.2 sudo apt-get install php7.2-mbstring