angular subscribe data json value

Data from server always comes as string. So, please parse it before using

Try

  this.http.post(link, myData)
       .subscribe(data => {
       data = JSON.parse(data);
      alert(data['code'])
   }, error => {
     alert("Oooops!");
  });

Just do console.log(data) and you will get to see why it is giving error.

As my guess is that it is a json response, and it has data.detail.code.

I was able to get it done by doing it in following way :

let dataJSON = JSON.parse(data["_body"]);

As per your error message it seems like you have passed wrong email address to API but if your email address is correct then you can try to check with below code as there is no need to use JSON.stringify if your API accept Content-type: application/json:

doLogin() {
  const link = 'https://www.sportsmanager.us/Ionic/ASPPages/IonicRMLogin.asp?Login=T&O=2';

  const myData = {
    txtEmailAddress: this.txtEmailAddress,
    txtPassword: this.txtPassword
  };

  this.http.post(link, myData).subscribe(data => {
    console.log(data);
  }, error => {
   alert("Oooops!");
  });
}

if your API do not accept Content-type: application/json then you have to add header in your POST API call as below :

this.http.post(link, myData, {
  headers: new HttpHeaders().set('Content-type', 'Content type accepted by API'),
}).subscribe(data => {
  console.log(data);
}, error => {
  alert("Oooops!");
});