2017 MacBook system upgrade download stalls

Try booting in safe mode and apply the update then.

Safe mode will often correct weird software behavior by forcing the OS to clear caches, do disk repair and other self-maintenance and housekeeping and temporarily disables third-party mods.

How to use safe mode on your Mac - Apple Support 

If you have installed any anti-virus, clean-up, security or VPN software, then you may need to disable it to update.

Also, disconnect any external devices - hubs, drives, etc. - before attempting to update.

Excellent suggestions by @D.I. Johnson.

Also, how much Free storage space do you have? Use Disk Utility to check on this Free space value since the "Available" storage value shown everywhere else within macOS is very misleading and is not synonymous with Free. You should have at least 20GB+ of Free space.