you need to use 'submit' button because there are no example teat cases

I can't see the error message.
I think you should copy these five testcase to the example testcase block.
and, should add some random test in the testcase block

I see, I see, just becase I'm so lazy    ;-)

@myjinxin2015: Please use proper formatting for code, e.g.

``` javascript
function answer(word){
  //your code...
}
```

will result in

function answer(word){
  //your code...
}

For more information, search for "GitHub flavored markdown".

You probably want to read the codex.

example for random test:
function answer(word){
//your code...
}
function randomword(){
//your code...
}
for (var i=0;i<100;i++){
var testword=randomword(), myanswer=answer(testword);
Test.assertSimilar(longestWord(testword), myanswer,'should return '+myanswer);
}

like this:
Test.assertSimilar(longestWord("The quick brown fox jumped over the lazy dog"), 6,'should return 6');
Test.assertSimilar(longestWord("1231231231231232131231231 ?!?!?!?!!?!? The quick brown fox jumped over the lazy dog"), 6,'should return 6');