If you haven’t corner rounding tool, you can use the following workaround. It’s useful only, if you do not need exact circular rounding with known radius:
add new anchor points into the red circles and remove the anchor point in the green circle. Use those anchor manipulation tools which are placed under the pen in the toolbox.
Inserted anchor point A was sharp because the path was perfectly straight. Now theres a new corner
Fix the sharp corner A by dragging with the anchor type conversion tool. A handle appears and you can drag it for a good fillet curvature.
In theory it’s possible that editing tne anchor point in the green circle gives the wanted result. For it other anchor points must exist near enough.