- laravel/framework v5.4.9 requires ext-mbstring * -> the requested PHP extension mbstring is missing from your system.

You can install it. What php version are you using?

on linux: sudo apt-get install php<version>-mbstring

on windows: http://php.net/manual/it/install.windows.extensions.php

I installed it, but i still have this issue

You need to go to the php.ini file and remove the ; placed before the extension.

If you are using a web server and not the php artisan command remember to restart the service