Enabling php ext-xml?
Try out Step 1:
sudo apt install php-xml
Step 2: Restart apache
Resolved my issue
It looks like you're using ubuntu 16.04 with the ondrej/php ppa to get the latest version of php. In this case, the package names need to match the version of php you're using:
apt-get install php7.2-xml
You can see the different versions available with something like this:
apt-cache search php | grep xml
Depending on your configuration, you may need to prefix the installation command with sudo
.