What would I do if my verification failed?

That can have different solutions depending upon the context in which you see it.

Use the document Add a payment method to your Apple ID - Apple Support to review your payment methods such as credit or debit cards. Problem methods will usually have a notice in red lettering below them. The article If your payment method is declined in the App Store or iTunes Store - Apple Support has some information about declined payment methods as well as a link at the end for contacting Apple about payment issues.

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

"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 - https://discussions.apple.com/message/33184357#message33184357 - "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