You need to indent line 28 by one more tab for correct formatting, as otherwise it is not part of the function.
For what to input into the exists function, it depends on the file path you are using to save the file. The first argument is the file path to the file you saved (you can ignore the second, I think).
For example: if you saved a file at “user://file.txt” then you would want the first argument to be “user://file.txt“. Likewise, if you saved a file at “res://saves/temp.txt”, then the first argument would be “res://saves/temp.txt“.