I don’t see anything. I can only think of a few things. The first is that the handle is already registered and you cannot override it. The second is that the space in “Open Sans” is causing a problem since this is used as an ID for the link tag and ID values cannot contain spaces.
Have you tried enqueuing the stylesheet without using the acf function, by just using
wp_enqueue_style( 'Open Sans', 'https://fonts.googleapis.com/css?family=Open+Sans:300,400,600', array(), false, 'all' );