Hi bigl2369, my guess is that you've probably got your rectangle as 4 separate lines instead of being joined together.

When you have totally separate curves the way fillet works is by doing only 2 curves at a time, so instead of selecting all 4 separate curves you would need to select just 2 and then do fillet, then select a different 2 and so on.

The faster way is to use Edit > Join to join your 4 separate lines together into one single curve. Fillet can handle all the corners of a joined together curve in one go, just select your joined rectangle and run Fillet, then it will ask you which corners you want to fillet, right click or push "Done" at that step without selecting any if you just want to do them all, then the radius you pick will get applied to each corner of the joined curve.

- Michael