Jmeter - POST API not working - api

Question

I am new to Jmeter and have been struggling to get it working to test my POST API. It works fine for a GET API call where I pass the parameters through the Parameters tab.
Details here:
server name: localhost
port: 8080
path /registerMobileUsingCode
In the request body, I am sending the following:
{
"clientName": "DemoOrg",
"code": "9880007615",
"languageId": "1"
}
My Jmeter setup looks like this:
And my HTTP Request Header looks like this:
The API is in production and works fine with postman.
I am just unable to get it working through JMeter. It is obvious that I am doing something wrong but can't figure out what despite having spent a considerable amount of time Googling for solutions.
Any help would be much appreciated.
UPDATE - 1 - Updated with jmeter log as suggested
Thread Name: Thread Group 1-1 Sample Start: 2017-08-09 17:03:41 IST Load time: 1604 Connect Time: 1525 Latency: 1604 Size in bytes: 399
Sent bytes:251 Headers size in bytes: 213 Body size in bytes: 186
Sample Count: 1 Error Count: 1 Data type ("text"|"bin"|""): text
Response code: 500 Response message: Server Level Exception
encountered
Response headers: HTTP/1.1 500 Server Level Exception encountered
Date: Wed, 09 Aug 2017 11:33:43 GMT Access-Control-Allow-Origin: *
Content-Type: application/json Transfer-Encoding: chunked Server:
Jetty(9.2.16.v20160414)
HTTPSampleResult fields: ContentType: application/json DataEncoding:
null
UPDATE-2 - Using Postman and JMeter Recorder
As suggested by #Dimitri T, I downloaded the Linux version of Postman and started it with --proxy-server=localhost:8888.
I was now able to make a Postman request and capture the request in JMeter. Here is what the request looked like:
Jmeter capture of a Postman Request
As you can see, nothing radically different from what I did. However, this DOES work! So hurray!

Answer 1

If your application works fine with Postman the fastest and the easiest way of building JMeter test plan will be just recording it using JMeter's HTTP(S) Test Script Recorder.
Prepare JMeter for recording (you can do it in a few clicks via JMeter Templates Feature)
from JMeter's main menu choose File -> Templates -> Recording and click "Create"
Expand Workbench - HTTP(S) Test Script Recorder and click "Start"
Prepare Postman for recording
In order to "tell" Postman to use JMeter as a proxy you need to provide --proxy-server parameter to it like:
C:\Users\YOUR_USER_NAME\AppData\Local\Postman\app-x.x.x\Postman.exe --proxy-server=localhost:8888
Execute your requests in Postman
JMeter will capture requests, convert them into HTTP Request samplers and store them under Test Plan -> Thread Group -> Recording Controller
References:
JMeter Proxy Step by Step
How to configure Postman Native Client App to use an external proxy

Answer 2

By seeing the image you have attached you are missing / in the path.
the path should be /registerMobileUsingCode
if it is working fine from POSTMAN, then your port 8080 is fine.
Can you please confirm what is the data type of your request? looks like languageId is integer so you should be using languageId: 1, instead of languageId: "1". The integer should not be double quotes.