Cannot implicitly convert type string to list - Why is this?

I am trying to write code that stores a list of strings, and then gets each one individually in a for loop. However, I keep receiving the following error:

 Cannot implicitly convert type `string' to `System.Collections.Generic.List<string>'

I have set the code to take the string only from the list but the error persists.

 List<string> textName = new List<string>();
 
 void OnGUI()
 {
     for(int i = 0; i < texts; i++)
         {
             string tn = textName[i];
            
             textName = GUILayout.TextField(tn); // Name - I have also tried tn.ToString() with the exact same results.
         }
 }

I don't understand why I am receiving this error.

Also, using a (string) cast doesn't work either.

Edit: Using .ToString() does not resolve the issue, and also using Arrays instead of lists produces a similar error involving the inability to convert string to string[].

This script is an editor script (extends from EditorWindow), could this be the cause of the issue?