Should look at the resulting length

Do that and you won't have a problem with that test, why do you say it hasn't enough Information about what's wrong?

A hashtag has no spaces, so it's not the input with a space which is called empty, it is the result:

If the input or the result is an empty string it must return false.

Also you don't check the length of the input:

If the final result is longer than 140 chars it must return false.

Please read the description again.