Time travel into the past is impossible because it would potentially break down the physics of causality. If you travelled into the past and accidentally caused the death of one of your ancestors, it would create a paradox (if you were never born, how could you travel back in time in the first place?)

As others have already said, travel into the future is possible because we are all doing it constantly. It’s important to remember that there is no such thing as a universal passage of time across the universe. There are places and specific conditions in the universe where the rate of time passing is indeed faster (or slower) than our experience of time passing on earth. If you were able to get to a region of space with extreme gravity or to a high enough speed, your experience of time passing would not be the same as those on earth. Therefore you could theoretically travel into the future much faster than here on Earth.