Always gets a blue screen for writing a specific IEnumerator method for a coroutine (Unity 2D, c#, script)

Question

Every time I finally get my IEnumerator method for a coroutine finally able to return a value, I get a blue screen and my hierarchy is empty. Is there something wrong with my code so I get a blue screen? If you know why, can you give possible solutions please?

Here's my method + other stuff pertaining the coroutine:

void Update()  
{  
    if (Input.GetKeyDown(KeyCode.Space) && playInfinite)  
    {  
        coroutineSFX = playSFX();  
        StartCoroutine(coroutineSFX);  
        Debug.Log("playInfinite is " + playInfinite);  
    }  
}  


private IEnumerator playSFX()  
{  
    if (answer.text.Equals(word.text + " ") || (answer.text.Equals(word.text))  
    {  
        playInfinite = true;  
        playDing();  
    }  

    else  
    {  
        playInfinite = false;  
        playBuzz();  
    }  

    yield return new WaitForSeconds(1f); //wait a second  
}  

I tried returning a bool variable, unsurprisingly, it did not work.

I tried converting the bool variable, playInfinite, into an IEnumerator but I couldn't because I couldn't add reference, SystemCore.Assembly, required for the using System.Linq. I honestly did not know what I was doing.

At last, I added a pause, and the current result is the above method + blue screen.

If you wish to help solve and you find the information is insufficient, please inform me and I will try my best to do so. Thanks for checking my post. :)