You probably need to set the architecture somewhere before booting (remember this from long ago), should be something in the menu when you create your system.
Are you getting the error on the host or the guest?
You need to have virtualization enabled in your computers BIOS. On Intel it is called VT-x and AMD calls it AMD-V, but different BIOS vendors can use different names (like Virtualization).
Make sure you have selected a 64-bit version of version template in VirtualBox Manager (Select your Haiku virtual guest machine β Settings β General β Basic β Version and set it to something 64-bit, like Other/Unknown (64-bit). This should enable point 3, but double check to be sure.
Enable virtualization on the VirtualBox guest machine (Select your Haiku virtual guest machine β Settings β System β Acceleration β Enable VT-x/AMD-V).
No other application can be active that uses VT-x/AMD-V at the same time VirtualBox is running.
Edit: The error message seems to be from the Haiku guest. So the guide above should fix your problem.
That guide do not mention the difference between setting up a 32-bit and a 64-bit guest machine.
We need to specify what kind of guest operating system we are going to install. This is helpful in the pre-configuration of virtual machines β it applies default settings for supported OSβs. For Haiku, we are going to choose βOtherβ for operating system type and βOther/Unknownβ for version:
This should be changed to something like:
We need to specify what kind of guest operating system we are going to install. This is helpful in the pre-configuration of virtual machines β it applies default settings for supported operating systems. For Haiku, set βTypeβ to βOtherβ. For a 32-bit Haiku installation set βVersionβ to βOther/Unknownβ, or for a 64-bit Haiku installation set it to βOther/Unknown (64-bit)β: