Ajax POST MyUrl 404（未找到）

i'm working on some grails code and geting this error whenever the ajax function is getting the response from the controller action!

the Ajax function is passing the parameters and controller function is performed after the return i get this error from the browser developing tool.

$.ajax({
    url: urlMap.myUrl_url,
    type: "POST",
    dataType: 'JSON',
    data: {
        pId: pId,
        stId: stId
    },
    success: function (data) {
        $supplementaryHolder.html(data);
    }
})

Here is the controller function !

def deleteSupplementary = {

    if (!modifyService.deletePlacement(params.long('pId'))) {
      flash.error = g.message(code: 'student.delete.placement.failed')
    }
     return params.stId;

  }

Mistake seems to be at this line

return params.stId;

change it to

render params.stId;