A story without a clear indication of success or failure is a failure with your readers or audience. You need to work out exactly how the audience will know the goal is achieved or not.
This might seem obvious in an action story, but may be much more difficult in a story about character growth.
Success and Failure don't have to be binary choices; they can be matters of degree. For example, the effort to bring back a treasure may fail, but the adventurers discover one large ruby that fell into their pack. Or, someone seeking true love might find love but with someone who is rather annoying.
Whether either of these examples is a partial success or a partial failure depends largely on how you portray the characters' attitudes to the imperfect achievement. To ensure a sense of closure in your readers/audience, make sure they know exactly how things end up on the success/failure scale.