laravel/framework[v5.4.0, ..., v5.4.36] require ext-mbstring * -> it is missing from your system. Install or enable PHP's mbstring extension. code example
Example: phpoffice/phpspreadsheet[1.1.0, ..., 1.15.0] require ext-gd * -> it is missing from your system. Install or enable PHP's gd extension
composer require "ext-gd:*" --ignore-platform-reqs