In Python, all indentation must be the same, for instance, if your first indentation is a 3 spaces, all indentations must also be 3 spaces. If the first indentation is a single tab, the rest must also be single tabs.
If though, one or more indentations aren't the same as all others, Python won't be sure if it's an indentation or not, so it will give this error. So make sure all your indents are correctly set.
:)