You can add fillets in both sketches and directly to edges of the model. Knowing when to use which will be a matter of experience.
You've gone for the sketch fillet, but you have a 3 way junction. you may wish to use the trim tool to remove the line segment not required. You can then simply use the sketch fillet tool on the corners.
Thank you i just trimmed the line where the slot and the square connects and i was able to do it