import qbs
import qbs.TextFile

Product {
    type: "custom"
    Transformer {
        Artifact {
            fileTags: "custom"
            fileName: "x"
        }
        prepare: {
            var cmd = new JavaScriptCommand();
            cmd.description = "creating file";
            cmd.sourceCode = function() {
                var f = new TextFile(output.filePath, TextFile.WriteOnly);
                f.close();
            }
            return cmd;
        }
    }
}

ERROR: Failed to create directory '/tmp/file-then-dir/Host-Qt5-stable-debug/project/x'