what do you mean by 'authenticate issue'?

Did you follow the exact key setting? like 1Ab2T4o?

oh. activation. with activated w8.1, and the computer connected to internet during the upgrade process, w10 will be activated automatically. you cannot use w8.1 key for w10 os.

did you change any hardware after upgrade?

if you have created a os clone, or system image, before you did the upgrade, restore the system to w8.1 with it, and try again. if not, you do a fresh install of w8.1, activate it, and then try again.

no.1 was the actual upgrade path. no. 2 will not work (incompatible key because of different os versions).

i think you should reinstall w8.1, activate it online or by phone, make a system image, then do the upgrade again.

you can use media creation tool to download the correct iso, make a bootable usb with it, and run setup.exe in windows to upgrade. don't disconnect your computer from internet when you upgrade.

Anybody who tries to use a Windows 8.1 key for Windows 10 will face the same problem as you.

There is no problem at all with upgrading Windows 8.1 to Windows 10 if you do it the proper way.

In Windows 8.1, you click on the setup.exe in the downloaded Windows 10 folder to start the upgrade process.

yes. make sure your w8.1 is activated first. the computer must be connected to internet during upgrade, starting from you clicking the setup.exe until you boot into new os successfully. once in new windows, connect to internet and it will activate itself.

good luck. hopefully this time everything will be fine.
many reasons. hardware still under validation; bad net; overloaded servers; etc. you can check out the faqs, http://www.microsoft.com/en-us/windows/Windows-10-faq

if you're tech savvy, just use media creation tool to do it manually.

that's the generic key for w10 home. i did w10 home for some machines previously. it worked :/

please confirm your windows version. maybe we have found the actual problem. (although it's weird that you could run setup.exe to upgrade. this is because it will show the product key dialog instead of starting upgrade process directly when the os do not match)

your windows 8 is Windows 8 Home or Windows 8 Home Single Language?  Check it at your Control Panel>System.

it's up to you. check out the posts here for ideas.
right. if you do not know what it does to your os, using these system tweaking software that involve windows registry is potentially unsafe. everything is done at your own risks. however talking about breaking things, how many times do we screw the os up just by using reg cleaners? the popularity of the tools may prove otherwise.

these days, reg cleaners are kind of user-friendly. they always create backups before committing changes. so if you're unsure, do not turn that backup function off. now what if i screwed up the os really big time? i just restore it from reg backups. if the damage is so beyond repairs, i just restore the os from a working system image, or do a reset, or a clean install.

the thing is, ms ships their os with default settings, but there is no such things as 'one size fits all'. in order to make a system run better, it's up to users to remove what are useless to it, and tweak the rest based on requirements.

of course ms will not support and is against the use. they have all the things they can say, i.e. when they provided their own tools to this, and when they decided to discontinue them.
what Cynox said.

it's up to you. check out the posts here for ideas.
right. if you do not know what it does to your os, using these system tweaking software that involve windows registry is potentially unsafe. everything is done at your own risks. however talking about breaking things, how many times do we screw the os up just by using reg cleaners? the popularity of the tools may prove otherwise.

these days, reg cleaners are kind of user-friendly. they always create backups before committing changes. so if you're unsure, do not turn that backup function off. now what if i screwed up the os really big time? i just restore it from reg backups. if the damage is so beyond repairs, i just restore the os from a working system image, or do a reset, or a clean install.

the thing is, ms ships their os with default settings, but there is no such things as 'one size fits all'. in order to make a system run better, it's up to users to remove what are useless to it, and tweak the rest based on requirements.

of course ms will not support and is against the use. they have all the things they can say, i.e. when they provided their own tools to this, and when they decided to discontinue them.
what Cynox said.

press win key + x, select command prompt (admin), type slmgr /dlv or slmgr /dli

please remove the previous screenshot as it contains your personal information.

.. and his email address. it's better to take it down lol
you have windows core edition, not single language. it's no wonder the w10 installer worked for you.

weird because the upgrade should work for you. my best guess about your w10 activation failure now is that it might be related to the channel type. i tried retail and oem (stock-key-in-bios type), both worked.