To enable extensions, verify that they are enabled in those .ini files - Vagrant/Ubuntu/Magento 2.0.2

First installed sudo apt-get install php5-gd, then sudo apt-get install php5-intl and last one was sudo apt-get install php5-xsl. After that, it's installing as it should.

This solved my issues

sudo apt-get install php7.1-xml

or

sudo apt-get install php7.2-xml

Have tried for many times, the above answers don't solve my quesiton, but this command helped me:

sudo apt-get install php-mbstring