A lost network connection means the connection was dropped. If you are using Wi-Fi, then switch to Ethernet. To avoid download errors you need a reasonably fast Internet service.
You may need to reinstall macOS:
Reinstall El Capitan or Later Without Erasing Drive
Please be sure you back up, if possible.
- Restart the computer. Immediately, at or before the chime, hold down the Command and R keys until the Apple logo and progress bar appear. Wait until the Utility Menu appears.
- Select Disk Utility and click on the Continue button.
- Select the indented (usually, Macintosh HD) volume entry from the side list.
- Click on the First Aid button in Disk Utility's toolbar. Wait until the Done button activates, then click on it.
- Quit Disk Utility and return to the Utility Menu.
- Select Install OS X and click on the Continue button.
