u need to install php-mbstring
package try.
check php version
php -v
then check mbstring already install and enable
php -i | grep mbstring
if not installed run this command
sudo apt-get install php-mbstring
if you are php other version example : 7.1, 7.2, 7.0 based on
run command like this :
sudo apt-get install php7.1-mbstring
if you are using nginx server for run laravel .. then check nginx configration file which version u have loaded in conf file..
go to cd /etc/nginx/sites-available
and open your configuration file..
if you are loading php7.2 version in nginx conf file..
fastcgi_pass unix:/var/run/php/php7.1-fpm.sock;
then u need to install 7.2 mbstring package..
sudo apt-get install php7.2-mbstring
and restart apache2 server
sudo service apache2 restart