'String'类型的值没有成员'substringToIndex'

I'm not sure what I'm missing here, but all I'm trying to do is get a substring from a string. I'm messing around in a playground, and my code is only three lines long lines long, I declare a string, get an index, then try and get a substring from it, and I get this error. This is all I have for my code...I'm not sure what I'm missing here, but all I'm