Hi Mala, sounds like you have it figure out now - yes if you give fillet a radius that is too large for it to fit in the area of your model it will just not do anything.

Sometimes especially when you have surfaces at a shallow angle the fillet can eat up quite a bit of space and so it can be a good idea to use a smaller and smaller radius (like keep halving the value) until you see what can actually fit in.

When you are at the radius stage you can either enter in a radius by typing it in (you can actually just start typing numbers immediately, you don't actually need to click on the radius field first, when you type numbers they go into the first numeric field for a command automatically) or by picking 2 points inside the viewport. If you use the pick 2 points method the distance between those 2 points will be used as the radius.

- Michael