I see several issues with your code:

1. You modify the input: the line 'string="isIsogram"' is changing your input. The string you iterate should be the one you get in the parameter.
2. You modify the iterated string from inside the loop: Regardless the fact that the assignment is unwanted, the location of the assignment, inside the loop that iterates the string is a (very) bad practice in general. Take care not to do that.
3. Algorithm efficiency (I home I'm not talking chinese here):
   • You iterate the string, which means the loop iterates all its characters (a total of n chars.)
   • Then, inside you use the built in 'count' function, which also iterates the string.
   • Thus, in the worst case (when the string is an isogram) your efficiency is O(n²), which is considered highly inefficient. Think of a way to improve that (you can get to O(n).)