IMAC OS upgrade

Also, ensure that you only use Safari to download the installer. It will likely fail if using Firefox or other browser.

You may also try making the upgrade while booted in safe mode. (hold the shift key down at startup). It can take much longer to safe boot (10 min) so be patient.

How to use safe mode on your Mac - Apple Support 

Safe mode disables loading third-party drivers, forces the OS to clear certain caches and logs and does other miscellaneous housekeeping. It's non-destructive to user data so is safe to do. Very often certain weird software behavior is corrected by simply logging in this way as a troubleshooting method. When you're in safe boot, the machine will not be at its best performance, especially with graphics, but that's expected.

EDIT - in addition, disable or uninstall any anti-virus, clean up or optimizing apps you may have installed as these can easily interfere with the upgrade process.

Delete any previously downloaded "Install macOS Mojave" app that currently resides in the Applications folder. Then empty the trash, download a fresh installer and try the process again.