When using WDS for deployment, there is no local install media. The boot & install WIM's are extracted from your ISO distribution and served remotely. All the Post-Setup files under "\sources\$OEM$" aren't available, and Setup never copies them to the install PC.
Under the WDS model, you're supposed to replace this with a post-install task to mount a folder share, and run an equivalent Post-Setup script.
1. Copy all the Post-Setup apps to a folder share.
2. Edit the ISO's "\sources\$OEM$\$$\Setup\Scripts\SetupComplete.cmd" and replace all file instances with network share paths.
3. Remove SetupComplete's last few lines which perform local script and folder cleanup. They're no longer required since all files are hosted from a share, and no local files exist.