I have not seen a definitive solution for your problem. So, I suggest you use my search link in trying to solve your problem: verification failed unknown error apple. You will see a number of links that are, or may be, related to your specific problem.
If you want to contact Apple Support to deal with your problem, see:
Get a call from Apple support to help solve your problem using this link https://getsupport.apple.com/?caller=kbase&PGF=PGF63005&category_id=SC0245&symptom_id=23362 Be patient. Sometime this site is not fully functional.
Apple ID Support https://support.apple.com/apple-id
Apple Support Twitter https://twitter.com/AppleSupport There may be a wait of an hour or so.
Apple ID account security for country-specific and region-specific help https://support.apple.com/en-us/HT204169
Get Support: We’re here to help https://getsupport.apple.com/. Go through the options to find the best one.