Are you trying to input this through a popup keypad, a handwriting tool, or a laptop/desktop keyboard?

I think that Khan Academy has eliminated the handwriting tool from the app platforms, but just in case your version of the app is still using it, here's what I used to do when I was using this tool in the iOS app on my iPad. If I remember correctly, it didn't even work for me when I had to do a cube root, but others here in the Support Community were able to get it to work. When I had to go to root > 2, I had to do it in the a^(1/5) format, which was usually accepted as the correct answer, unless the specific skill being tested was using the root/radical sign. When this happened, I switched to the web version and did the exercise in my browser, where I got the popup keypad and, if I was on my laptop, could also input answers via the computer's keyboard.

As for using the keypad, I don't have the layout right in front of me, so I'm going to do this from memory. When the keypad pops up, look on the left side of it. You should see three keys with the radical sign. One does not have a superscript in the top left corner; that's for a square root. One has a "3" superscript in the top left corner and is for cube roots. The third one has an "x" superscript in the top left corner. Choose this one. Then, your input answer box will have a "focus" in the spot of this "x". Type your digit such as "5". To move on from where you've typed your digit, look for arrows to navigate beyond this spot or click your mouse where it should be.

If you're using your computer's keyboard, I suggest trying popup keypad entry as described above. I don't have much experience using the keyboard method, but maybe someone else here can advise you on this.