You are trying to resize the Virtual Hard Drive of Boot Camp based virtual machine from Configure -> Hardware -> Hard Disk 1 -> Edit but option is not available.
Boot Camp based virtual machine cannot be resized from virtual machine configuration window because Boot Camp based virtual machine uses a real Hard Drive partition where your Boot Camp is installed.
Select a Boot Camp-based virtual machine and choose Import Boot Camp from the File menu. Or you can right-click this virtual machine in the Window menu -> Control Center list and choose Import Boot Camp from the context menu -> click Import.
Note: This virtual machine must be shut down.
In the next step, locate where you want to store your new virtual machine and click Choose. Wait while the Boot Camp data is being imported to the virtual machine. After that, you will be able to use the newly created virtual machine in Parallels Desktop.
Note: You will be able to work with the original Boot Camp virtual machine as well.