When selecting the MSI package, the Windows Operating System type is grayed out and it only shows 32-bit.
Learn how to create a 64-bit MSI package using the Client Packager.
Selecting to create an MSI Client Package bundles both the 32-bit and the 64-bit drivers into one. This explains why the Windows Operating System type is grayed out.
The newly created MSI package automatically detects the system it is being installed on, and installs the 64-bit engine when it detects a 64-bit system. There will only be one MSI package, but it will contain both installation packages.
Please check the properties of the Virus Scan Engine component. It should show the Virus Scan Engine (64-bit) on a 64-bit machine.