I am using the auto desktop page size, and i fit all elements inside, it is my front login page and it still is scrolling… any idea why… and how can i fix i? 
What do you mean by auto desktop page?
Prototype overflow scrolling property is responsible of the children layers of the selected frame. Meanwhile, your main frame size might be larger then your screen size, or the screen you’re showing the prototype on.
This will lead your prototype to scroll until reaching the edges of the frame (Horizontal or Vertical).
To solve this make sure that your main frame size is a little bit less in height than your screen size (to account for the hight of the browser top bar or figma app bar -the bar containing the tabs-)
Hello,
Please check that the content of the page is contained within the bounds of the viewport. If any content is overflowing beyond the boundaries of the viewport, that may cause the page to scroll. To check for overflow, select the artboard in Figma, and check the properties panel to see if the overflow property is set to “scroll” or “auto”. If it is, change it to “hidden” or “visible” to prevent scrolling.