Установите php-mbsstring в Ubuntu
sudo apt-get install php7.2-mbstring
sudo apt-get install libapache2-mod-php7.2
sudo service apache2 restart
Lokesh003Coding
sudo apt-get install php7.2-mbstring
sudo apt-get install libapache2-mod-php7.2
sudo service apache2 restart sudo apt-get install php7.4-mbstring
sudo apt-get install libapache2-mod-php7.4
sudo service apache2 restart