I have not seen a definitive solution for your problem. So, I suggest you use my search link in trying to solve it: account is unavailable apple. You will see a number of links that are, or may be, related to your specific problem. Some claim fixes.
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.
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But, note that your situation may be connected with Activation lock. If it is, you apparently acquired the phone second hand. The following information explains what has happened. You apparently didn't know to heed the advice given below.
"Before you buy/acquire an iPhone, iPad, or iPod touch from ... [a third party], make sure that the device is erased and no longer linked to the previous owner’s account."
And see: Activation Lock for iPhone, iPad, and iPod touch https://support.apple.com/en-us/HT201365.
See this heading Check for Activation Lock before you buy/acquire a device from someone else.
The presence of activation lock means that we can't help you. We are not authorized to remove the activation lock from any device. And, under your circumstances, Apple support will not remove it since you
cannot provide proof of ownership that they will consider to be sufficient. You would need and have your purchase receipt for the device from an authorized Apple seller with the device's serial number on it.*
If you can't contact the prior owner and don't have the receipt, the device is useless to you.
*Rarely, an iDevice is purchased activation locked from an authorized Apple seller.
