They literal curly brackets in that GUID need to be escaped.
Because you are already in a Relevance Substitution, you do not need to escape the open-bracket. But to stop the close-bracket from ending the Relevance Substitution early, you do need to escape that one as }}
See Tip: Escaping curly brackets for substitutions in ActionScript
And try
waithidden {value "UninstallString" of key "HKEY_LOCAL_MACHINE\SOFTWARE\Microsoft\Windows\CurrentVersion\Uninstall{D6932D97-36F1-40B8-9CDC-CA8365B21000}}" of registry as string}