Composer Install - requires ext-mbstring

Try installing mbstring php library with aptitude.

sudo apt-get install php-mbstring php7.1-mbstring

php apache composer-php cpanel mbstring

Installing rh can solve this issue yum install rh-php56-php-mbstring.x86_64

Please double check what you have in your server. Mine is rh-php56-php-mbstring.x86_64 not php-mbstring

php apache composer-php cpanel mbstring

Use this command to install mbstrig

 yum install php-mbstring

Once installed restart apache

 service httpd restart

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