Verification Failed An unknown error occurred.

If you can't connect or sign in to iCloud, read this support article --> If you can't connect or sign in to iCloud - Apple Support

"Verification Failed This Apple ID is not active" --> According to Apple Community Specialist Brittany1416 in thread Inactive Apple ID - Apple Community , please contact Apple Support for help with an "inactive" Apple ID.

You can reach Apple Support using this link --> Contact - Official Apple Support

or by telephone at one of the numbers in this support article --> Contact Apple for support and service - Apple Support

"If you can't connect to the App Store, iTunes Store, or other Apple services" click here --> If you can't connect to the App Store, iTunes Store, or other Apple services - Apple Support

- If none of the suggestions work, try the last one: "If you’ve tried all of the above steps and you still can’t connect, contact Apple Support."

If you see a message saying to try again later, see if it's a system issue: Apple - Support - System Status

Verification Failed "problem connecting to the server" March 2018 - Verification failed - Apple Community - Needed to connect over wi-fi and not through cellular.

And the opposite:

"Verification Failed, An unknown error occurred" March 2018 - update apple ID settings notification in … - Apple Community - "tried again with Wifi off and it worked"

If those don't resolve the problem, I suggest you contact Apple:

Contact Apple. Click here --> Choose your country or region - Official Apple Support Select your country, then a product. If you don't see one that handles your issue then keep experimenting with selections until you reach one that gets you a chat session or a telephone call and get the representative to redirect you.

or:

Contact Apple for support and service --> "See a list of Apple phone numbers around the world."

Click here --> Contact Apple for support and service - Apple Support