Thanks for reaching out to the Apple Support Communities.
I see that you are having an issue with the iCloud Backup completing on your new iPhone 8. I'll be happy to provide some help.
First, please try the steps in this support for help backing up to iCloud:
If iCloud Backup couldn’t be completed or you can’t restore
Then, once the iCloud Backup is complete, this support article will help you update:
Update the iOS on your iPhone, iPad, or iPod touch
Let me know if you have any questions.
Cheers.