Why can we not create a time machine so we can go back in time? I saw that we have the parts but why can't we make it?
There are many theories on time travel and if it is possible. The prevailing thought on the matter is that it is not possible to travel backward in time. We know through Einstein's special theory of relativity time slows or speeds up depending on one's relation to an observer. For example, the closer to the speed of light a person travels (in reference to an unmoving observer) the slower they age. Experiments have been performed which suggest gravity has an effect on time as well.
Although theories on time travel (backwards) abound, we do not currently possess the technology to undertake any of them in experimental form. According to Einstein, as matter gets closer to the speed of light it gets heavier, therefore requiring more energy to reach light speed. He surmised it would create a feedback loop where the mass would increase with speed, and light speed could not be reached. People have tried to use Einstein's theory of special relativity to argue time travel might be possible. However, the consensus is the gravity necessary to generate the complex wormholes would require much more energy than we can produce and exotic matter that may not exist.