If time-travel is possible, then where are all the time-travellers? If time travel is possible, then we might expect future time-travellers to have arrived in our time. Or, more formally:
If time-travel is possible, then at some point in the future, humans will time-travel.
If humans will time-travel, they will travel back to our time.
If humans will travel back to our time, then we should have encountered them.
We have not encountered any time travellers.
Therefore, time travel is not possible.