Android studio: Retrofit2 POST request not working - android-studio

Question

So, when i make a post request to my api, it run the onFailure fun. The logcat doesn't give an error. Why is that? Im new to android studio.
Can you help me? Thanks!
Where i send the POST: http://localhost:3000/test
Response (Request sent with postman):
{
"cso": "kobasz!"
}
Heres my code:
Main activity:
package com.alms.chatenium
import android.annotation.SuppressLint
import androidx.appcompat.app.AppCompatActivity
import android.os.Bundle
import android.util.Log
import android.widget.Button
import android.widget.TextView
import android.widget.Toast
import retrofit2.Call
import retrofit2.Callback
class MainActivity : AppCompatActivity() {
override fun onCreate(savedInstanceState: Bundle?) {
super.onCreate(savedInstanceState)
setContentView(R.layout.activity_main)
val button = findViewById<Button>(R.id.testbtn)
val textView = findViewById<TextView>(R.id.textView)
button.setOnClickListener{
Toast.makeText(this#MainActivity, "TEST", Toast.LENGTH_SHORT).show()
val retrofit = ServiceBuilder.buildService(APIInterface::class.java)
val obj = RequestModel("djalms")
retrofit.requestTest(obj).enqueue(
object:Callback<Response>{
override fun onResponse(
call: Call<Response>,
response: retrofit2.Response<Response>
) {
textView.text = response.body()?.cso.toString()
}
override fun onFailure(call: Call<Response>, t: Throwable) {
textView.text = "Fail!"
}
}
)
}
}
}
Interface:
package com.alms.chatenium
import retrofit2.Call
import retrofit2.http.Body
import retrofit2.http.POST
interface APIInterface {
#POST("test")
fun requestTest(#Body requestModel: RequestModel): Call<Response>
}
Request model:
package com.alms.chatenium
data class RequestModel(
val username: String
)
Response model:
package com.alms.chatenium
class Response (
val cso: String
)
Service builder:
package com.alms.chatenium
import okhttp3.OkHttpClient
import retrofit2.Retrofit
import retrofit2.converter.gson.GsonConverterFactory
object ServiceBuilder {
private val client = OkHttpClient.Builder().build()
private val retrofit = Retrofit.Builder()
.baseUrl("http://localhost:3000/")
.addConverterFactory(GsonConverterFactory.create())
.client(client)
.build()
fun<T> buildService(service: Class<T>): T{
return retrofit.create(service)
}
}
API tested with postman, and verified, that the api (NestJS) working.