Updated my system OS and now I get a code error?

I don't understand this. I don't think it has to do with my system update but I am running a coroutine that calls this method below to delay a couple seconds.

Code (CSharp):

1.     IEnumerator waitTimeToFire(){
2.         yield return new WaitForSeconds(2f);
3.         calculateAttack(theWind, castleXPos);
4.     }

And suddenly this error pops up.

Before coming here for help I did a search on the error message which came up often but the solutions don't seem to work? Solutions seem to imply changing NameSpaces would instantly solve this problem, it did not. Then I added a return type like

Code (CSharp):

1. IEnumerator<float> waitTimeToFire()

This solved the error on that first line but pushed the problem down 1 level and gave me an invalid return type in the waitForSeconds().

That gives me this error.

My current name spaces are

Code (CSharp):

1. using System.Collections.Generic;
2. using UnityEngine;
3. using UnityEngine.UI;
4. using System.Linq;

So I am running in circles here, any ideas?

 

try adding System.Collections
Did happen to me. Might be a bug or whatever, but it worked.

 

This is so ridicules. This was one of the solutions that I tried yesterday and it didn't work. But today I tried it again and it worked. Thanks.

I don't get it thought? I did a OS update on my Mac yesterday and then after I updated to 10.12.6, from 10.12.5 this error message popped up. Thanks!