while downloading the OS, shown connection occurred error, but the internet connection is found no problem. Why?

Hello carolmo,

Thank you for reaching out in Apple Support Communities. We understand you're unable to update the software on your Mac, due to an error message that the network connection was lost.

Having a seamless experience in updating the software on your Mac is important, and we'd like to help.

If you haven't tried yet, we recommend to restart your Mac in safe mode. This helps perform a basic check of your startup disk, deletes some system caches and more. Here's a helpful article with additional information and the proper steps to take: How to use safe mode on your Mac

You'll want to follow the sections within the article that says, "if the issue continues in safe mode" and "If the issue doesn't continue in safe mode."

In addition, you can try on another Wi-Fi network to see if you can update the software.

Please keep us posted with the results.

Kind regards.