There are factors:
Some which are beyond your control and those with are totally under your control
Software on the computer interfering with the download process including AntiVirus Software
Notation: Disabling the AV Software may or may not solve the issue.
Removal of the AV Software may increase the chances of the download to perform as expected.
Running behind a VPN which may increase Packet losses or corruption of the download
Turn off any Third Party Two Way Firewall and try again
Best Results are via Ethernet Cable to Router
Restart in Safe Mode. This will perform a Disk Repair, clear cache files and only load Apple Software, extensions and fonts.
The boot up will be slow and can take some time - Normal
Try Again using the Software Update Applet in System Preferences / Setting
Worst case - Restart in Recovery Mode and choose Reinstall macOS.
This in theory will pull the latest version of macOS this computer qualifies to run