Illustrator Live Corners is a 17.0.0 version feature.
Instead you can always use Round Corners from menu Effect > Stylize
Adobe Illustrator CS6: Rounding One (or Two) Corners
Select a rectangle.
Select the Scissors tool and click two opposing corners to separate the single rectangle > into two independent right angles
Select one of the right angles, and use Effect > Stylize > Round Corners to round one corner
Choose Object > Expand Appearance
Repeat steps 3 and 4 on the other right angle.