It’s a very subtle thing, i couldn’t tell if the punctuation was part of the test, or part of the result. Add a missing punctuation mark to the end of your sentence.
This isn’t really anything specific to programming, it’s just that the instructions want a period at the end of that sentence, so you need to add a period.
And I agree with @snowmonkey, the period is very hard to see in the example (it appears to be colored light green).