the requested PHP extension mbstring is missing from your system
-- EDIT BASED ON NEW INFORMATION --
Since this is Windows, re-installing PHP, enabling the extension in the PHP.INI file, and restarting the Webserver, should work.
-- PREVIOUS SOLUTION FOR LINUX --
You need to install mbstring for php.
Use:
sudo apt-get install phpX.X-mbstring
Where X.X is your version number of PHP.
Its worked for me:
sudo apt-get install php7.2-mbstring
It works
sudo apt-get install php7.3-mbstring
sudo apt install php-xml