OK. There are specific use cases as @Nachlese suggested, where one GPU could work, however, to use GPU passthrough in the way you describe you need two (2) GPUs. Period. Usually one iGPU (such as onboard Intel graphics) and one discrete dGPU (such as your RTX 3060).
In an example like that your RTX 3060 could remain as the main system GPU and the Internal graphics could be used (on some systems) for another purpose, such as purely math calculation (with no monitor attached), or as a GPU passthrough situation for a VM.
A virtual machine doesn’t do this by default, Instead, it uses its own Graphics driver and uses a portion of system RAM to enhance video performance. This does not use your graphics card directly.
In the guide you linked, it states: “Passing through your primary GPU from your host OS to a Windows 10 VM” – Note the word primary – this also suggests there should be a secondary GPU. If you do not have a secondary GPU (as you claim) then you might be out of luck.
In any case, you need to educate yourself on Virtual Machines, and probably Manjaro too, to the extent that you can confidently ask a question that others will understand, and understand answers you are given without throwing a tantrum.
Further to this, there are multiple comments in this thread that you have not even acknowleged. I fail to see how you expect to receive any assistance if you will not observe that simple courtesy.