This is kinda weird seeing as I have used " for all the other ones, and I’m now wondering if there even is a difference, and why won’t it let me use " anymore?
there is no difference, you can use both (except when an exercise is being annoying)
the only difference is that if you want to use quotation marks in a string, you should enclose the string in apostrophes (and vice versa) so you don’t have to escape the quotation marks