MVC Web API, Error: Can't bind multiple parameters

Reference: Parameter Binding in ASP.NET Web API - Using [FromBody]

At most one parameter is allowed to read from the message body. So this will not work:

// Caution: Will not work!    
public HttpResponseMessage Post([FromBody] int id, [FromBody] string name) { ... }

The reason for this rule is that the request body might be stored in a non-buffered stream that can only be read once.

emphasis mine

That being said. You need create a model to store the expected aggregated data.

public class AuthModel {
    public string userName { get; set; }
    public string password { get; set; }
}

and then update action to expect that model in the body

[HttpPost]
public IHttpActionResult GenerateToken([FromBody] AuthModel model) {
    string userName = model.userName;
    string password = model.password;
    //...
}

making sure to send the payload properly

var model = { userName: "userName", password: "password" };
$.ajax({
    cache: false,
    url: 'http://localhost:14980/api/token/GenerateToken',
    type: 'POST',
    contentType: "application/json; charset=utf-8",
    data: JSON.stringify(model),
    success: function (response) {
    },

    error: function (jqXhr, textStatus, errorThrown) {

        console.log(jqXhr.responseText);
        alert(textStatus + ": " + errorThrown + ": " + jqXhr.responseText + "  " + jqXhr.status);
    },
    complete: function (jqXhr) {

    },
})