While ad takeover of Windows Server 2012 Active Directory you receive:
This means that the ad domain and forest function level are set to 2012. Unfortunately it is currently not possible to takeover these without additional preparation steps.
You can simply lower the function level of forest and domain with the help of powershell.
Simply do this in powershell at your Windows Server Active Directory domaincontroller to lower the levels to 2008R2:
Set-ADForestMode -Identity "mydom.local" -ForestMode Windows2008R2Forest Set-ADDomainMode -Identity "mydom.local" -DomainMode Windows2008R2Domain
Afterwards the takeover should be possible.