Successfully importing data into WordPress is not something we can guaranty for all users. There are a lot of variables that come into play, over which we have no control. For example, one of the main issues are bad shared hosting servers.
General error messages you might get while trying to import the demo content:
Server error 500
You clicked on the “Import” button and the response from the server was something along the lines of:
Server error 500 Internal server error (500)
This usually indicates a poor server configuration, usually on a cheap shared hosting (low values for PHP settings, missing PHP modules, and so on). Please check with the Theme System Requirements.
Server error 504 – Gateway timeout
This means, that the server did not get a timely response and so it stopped with the current import. What you can try is to run the same import again. If you get the same error, you can try to run the same import a few times. A couple of import tries might finish the import until the end because your server will be able to process the import data in smaller chunks.
Missing PHP modules
Fatal error: Class 'DOMDocument' not found in ...
That means, that your hosting server is missing one common PHP module and it has to be enabled before any import functionality will work on your site. The missing PHP modules are: php-xml or/and php-dom. Please contact your hosting company and ask them to install that for you.
Fatal error: Class 'XMLReader' not found in ...
Similarly to the above, this time the XMLReader PHP module/extension is missing. Please contact your hosting company and ask them to install that for you. These are very common modules.
Please open a support ticket if you continue to experience issues.