Help with Fill Tool

Support Basics & Interface
1

Hi
So i recently started using Blender and i am having problem with fill tool
Everything will explain the image below:

screen.png1440×838 123 KB


On right side things are fine but on left idk what happened there.
Should be same stuff because on left side is mirror modifier,i saw this on videos on youtube.
Link for video:https://youtu.be/7aU6Wty9fvg

2

I assume used shortcut F to create a non planar face which can be appear in numerous ways in the viewport. This is not the fill tool. The fill tool is Alt+F.

Where in the video does it say to fill this area. I saw where it told you to extrude (shortcut E) but not fill