I am trying to use ArcGIS print services to print maps as pdf by using the Url from the ArcGIS server. I want to print the map and all visible layers using an Export Web Map Task (PrintingTools Service) from ArcGIS Server. I am passing my data as JSON string to execute URL and in return will get the URL of the PDF for downloading. here is my code

 var spec = {
            "Web_Map_as_JSON": JSON.stringify(web_map_json),
            "Format": this.selectedFormat,
            "Layout_Template": this.selectedLayout,
            "f": "pjson"
        };
        var uri_spec = $.param(spec);
        var url = this.map_service.printExecute_url;
        var http = this.$http_({
            method: "POST",
            url: url,
            headers: {'Content-Type': 'application/x-www-form-urlencoded'},
            data: uri_spec
        });
        http.success(function (data) {
            console.log(data);
            this.printState = false
        };

The problem is that I keep getting this below error :

error:
code: 400
details: ['Error executing tool. Export Web Map Task : error …eb Map).\nFailed to execute (Export Web Map Task).']
extendedCode: -2147467261
message: "Unable to complete operation."

I have also referred to this similar questions https://gis.stackexchange.com/questions/119292/unable-to-print-feature-layer-from-my-arcgis-server but the solution doesn't work for me. Please let me know if there is any solution to this issue. Thanks in advance.