Application schema support is a GeoServer extension and is downloaded separately.
Download the app-schema plugin zip file for the same version of GeoServer.
Unzip the app-schema plugin zip file to obtain the jar files inside. Do not unzip the jar files.
Place the jar files in the
WEB-INF/libdirectory of your GeoServer installation.
Restart GeoServer to load the extension (although you might want to configure it first [see below]).