I don't understand what google wants to tell. I want to learn sticky ads is free or not under the page on mobil devices. Google alert. What google wants to tell there? Is it free to use sticky ads like this page?
It's just simple. You've created an Ad unit of 300x250 and the unit does perform well. But since Google gives a lot more importance to mobile friendliness, it is suggesting you to create an ad unit of responsive type - so that your ad size automatically will be adjusted to the screen accordingly (on mobile and PC/Laptop).