Error in calling POST method on fetch method

Question

I am receiving below error when calling my WebAPI from react application using fetch().then().then().

415 Unsupported Media Type

React code:

function handleSubmit() {
console.log(selectedClass, textAreaData);
let classInfo = { className: selectedClass, attendenceData: textAreaData };
fetch("http://localhost:58882/GenerateExcel", {
  method: 'POST',
  mode: 'no-cors',
  headers: {
    'Content-type': 'application/json; charset=UTF-8'
  },
  credentials: 'same-origin',
  body: JSON.stringify(classInfo)
})
  .then(response => response)
  .then(data => {
    console.log(data)
  })

}

WebAPI:

[Route("GenerateExcel")]
    [HttpPost]
    public IActionResult GenerateExcel([FromBody] ClassInput input)
    {
        string path = "";
        string selectedClass = "6"; string attendance;
        StringBuilder sb = new StringBuilder();
        switch (selectedClass)
        {//some logic}
}

public class ClassInput
{
    public string ClassName { get; set; }
    public string AttendenceData { get; set; }
}

This works fine when its GET.