You are trying to import a new Gold VM to be an OS layer.  The first thing you need to do is create a Connector.  In the Connector, you see the field for Template, and you try to select your Gold VM there.  When you test it, it fails saying one of these:

"The virtual machine template cannot have any attached disks. Please detach the disks through your vCenter console."

"The virtual machine cannot have virtual disk attached."

"The template cannot have virtual disk attached."

When you're creating a Connector for vSphere, XenServer and Nutanix AHV, there's an optional field for VM Template. This is not for your gold.  This is so we can create temporary machines in the future for layer editing; or create permanent machines if you want to publish an Image straight to the hypervisor. Basically, any time we want to make a machine for any purpose, we'll clone that template. Because we'll supply our own disks after the fact, that template needs to have no disks of its own.

It's optional, but it's worth doing for consistency's sake, so you might in fact clone your Gold VM (if it's configured in the same way you expect your end-user machines will be), delete the disks from the clone, and convert that to be a template for us. Or (in vSphere) you can ignore it for now, and some day in the future, give us a template to use by editing the Connector. If you don't specify a template in vSphere, we will create a machine when we need one with a default of 8 CPUs and 4GB - or something enormous like that.

The part where you actually import the Gold VM comes well after that. Create the Connector, specifying the VM Template if required, and close that process after you've saved. The next tab in the wizard back in the Management Console web page will then use that Connector to talk to your hypervisor and get a list of proper VMs (not templates), which you can use to pick the VM that you want us to import. That VM should indeed have its boot disk still attached, and should not be the Template VM you configured before.