Can I skip the downloading of the Windows support software if I already have it? Yesterday I was facing a problem that Boot Camp Assistant was stuck on copying Windows files but that problem is solved now. After that copying files Boot Camp Assistant is showing that it is "Downloading Windows support software". I have already downloaded this Windows support software on my MacBook, therefore is there any way to skip this process?

Unticking the option is grayed out unfortunately.
If you are creating a Windows install disk then you can't unselect the option to download the Windows support software. Boot Camp Assistant, whilst creating the boot disk, will copy the relevant support software into the install media. This process requires a copy of the Windows support software which it will download from the internet.

There is currently no way to use your own copy of the support software if you create a boot disk. If you already have the boot disk, you can just select the option to install Windows, in which case you can use your own copy of the Windows support software.