Sounds like you have not built/downloaded the prerequisites yet. Have a look at https://github.com/trodemaster/packer-macOS-11#prerequisite-installer-bits
Apple being Apple does not simply post nice .iso files of their OS and compiler publicly. The prerequisites script wrapps a couple other mac comunity tools which generate the needed installer .iso and create a checksum. If you want Xcode then you will need to download them from developer.apple.com and put the files into the install_bits dir firs.
This template has three builders inside it. The main one full does the full build of the OS including installing Xcode. With the base build it’s just the OS and VMware tools that get installed. When you run the customize build it starts with the output of the base build. If you have never built the base then the errors about missing .vmx files with show on validate.
If you get success with the scripts/buildprerequs.sh then run
packer build -only=base.vmware-iso.macOS_11_base macOS_11.pkr.hcl
you should be off to a good start.