Certain iPhone users have been plagued lately by seeing an iOS Verification Failed message when trying to login on iCloud.com. Apple has acknowledged the error and has talked with several people on this.
Having spoken to Apple support and been escalated to a senior team member, it seems that Engineering are aware of a limited number of users that are also experiencing this.
Apple has a System Status page where you can check to see if certain services are experiencing issues and downtime.
If the issue hasn’t already been fixed, it seems that the only thing to do for now is just…wait.