You can easily use the feature of body to create the nut with the fillet as the above figure
First of all create a hexagon and a circle in the sketch on XY plane & Pad it to look like this.
After that go to INSERT -> BODY.
Now sketch a circumscribed circle for the hexagon on the XY Plane
Pad the circle to become a cylinder for the same pad length as the hexagon
Now create a fillet on the cylinder for the required dimension like below.
Go to INSERT -> BOOLEAN OPERATORS -> INTERSECT as shown in figure
Select the cylinder first, then after select the paded hexagon, & then click ok to look like this.
Your required Hexagon nut looks like this. See here.