Many users with MacBook Pro 2018 models with the T2 chip are reporting that upgrading to macOS 10.14 Mojave fails towards the end of the process.

Recommendations which have proved successful for some include resetting the SMC prior to upgrading, or reinstalling High Sierra in Recovery mode before trying the upgrade. If you experience problems, you would be wise to contact Apple Support for their latest solution.

(Thanks to MacFormat for drawing attention to this support community thread.)