Composer - the requested PHP extension mbstring is missing from your system

sudo apt-get install php-mbstring

# if your are using php 7.1
sudo apt-get install php7.1-mbstring

# if your are using php 7.2
sudo apt-get install php7.2-mbstring

# if your are using php 7.4
sudo apt-get install php7.4-mbstring

For php 7.1

sudo apt-get install php7.1-mbstring

Cheers!

1. find your php.ini
2. make sure the directive extension_dir=C:\path\to\server\php\ext is set and adjust the path (set your PHP extension dir)
3. make sure the directive extension=php_mbstring.dll is set (uncommented)

If this doesn't work and the php_mbstring.dll file is missing, then the PHP installation of this stack is simply broken.