Ambiente: Ubuntu 17.10 (GNU/Linux 4.13.0-43-generic x86_64)    
Servidor: Apache/2.4.27 (Ubuntu)    
PHP: Version 7.1.17

When you start the Magento 2.x platform installation process, an error known in Step 2 will make it impossible to move.

The reported error:

  

Notice: undefined property: domdocument :: $ documentelement in   /var/www/html/sualoja/vendor/magento/framework/objectmanager/config/mapper/dom.php   online 58

The reasons:

1 - Lack of permission on directories and installation files.

2 - dom.so extension not enabled or double-enabled in php.ini

The solution:

First of all, make sure the php-xml extension is installed. Use the command in the Ubuntu terminal to install:

sudo apt-get install php-xml

1 - Change the permissions of directories and files on your server

chmod 777, in the directories:

app/code
app/etc
pub/static
var/generation
var/di
var/view_preprocessed

2 - Search the php.ini files on your server with the command:

find / -name php.ini

It can be found at:

/etc/php/7.1/cli/php.ini

/etc/php/7.1/apache2/php.ini

Enable extension=dom.so in only one of the php.ini files

You can do this using the command:

sudo nano /etc/php/7.1/apache2/php.ini

  

Use CTRL+W to find the line of code extension=dom.so .   Enable the extension by removing the bullet point . Change% with% to    ;extension=dom.so . Save extension=dom.so and close CTRL+O the file    php.ini when done.

Restart the Apache server for the new changes to take effect:

sudo systemctl restart apache2

Resume the installation and verify that the problem has been fixed.