Fix "Unable to download the operating system installation files."

Symptom

When trying to download an operating system using the Installation Assistant, the following notification appears:

Unable to download the operating system installation files.

Resolution

Note: to download Windows 10 installation image manually visit this page. To install Windows, follow instructions from KB 123284

1. Check your Internet connection. If you are using Wi-Fi or mobile network, connect the computer to the Internet using Ethernet cable to ensure network stability.
2. Do not close the lid of your portable Mac when downloading.
3. Click OK on the error message and click Resume.
4. Try downloading the operating system again.

Advanced Troubleshooting

1. Try accessing this page. If the page is accessible, follow the steps below. If you cannot access it, try later.

2. Open Terminal.app (Finder → Applications → Utilities) and execute the command:

   ping download.parallels.com
   

3. Make sure that the address is resolved and you receive response from the server.
   
   
   
   Press Control + C combination on the keyboard to stop the process and see the results.
    
4. If you cannot connect to download.parallels.com check DNS settings and make sure that port 443 is opened for TCP protocol (Firewall settings).