This past week, 'Windows Insiders' were treated to quite a few builds of Windows 10 Technical Preview. But some users are experiencing issues with their builds as they are unable to activate the OS.

Currently, Microsoft is experiencing some issues on the back-end with Windows 10 activations. According to Microsoft, they are currently doing some testing that will cause activation failure for the next couple of days. Microsoft is urging users that have this issue to ignore the message. The problem should rectify itself, once the activation service is restored.

While we don't know how widespread the issue is, it seems as though those using builds 10159 and 10162 are having this issue. Interestingly, there seems to be a newer build available, build 10163, although it has not been released via official sources.

Windows 10 is set to arrive on July 29th, 2015. The Windows 10 update will be free for those running eligible copies of Windows 7, 8 and 8.1.

Source: Microsoft | thanks for the tip xendrome !