Always On Availability Groups on Windows 2012 Standard I am trying to enable the Always On Availability Groups feature of SQL Server 2012 Enterprise Edition on Windows Server Standard Edition 2012, but it says the feature requires X64 Enterprise Edition of SQL Server 2012 or Windows 2008 Enterprise edition or above.

I already have Windows Standard 2012 and SQL Server 2012 Enterprise Edition, why can't I enable this feature?
Per the information you provided from SELECT @@VERSION you are running the x86 version of SQL Server 2012. It would appear that the OS and SQL Server architecture must match in order for Availability Groups to function.

This means that you would either need to change your SQL Server installation to the x64, or go to a prior version of Windows (2008) and install the x86 version of that.

My recommendation would be to adjust your existing install and make it x64. This provides many enhancements such as not needing to mess with things like PAE and AWE when using more than 4GB of memory.