As far as science fiction goes, time travel is likely never, ever going to be possible. If it is, we're still likely tens of thousands of years away from discovering it.

Invisibility, however, is not. Why haven't we mastered invisibility? As in, the ability to make a cloak that is perfectly invisible?

No, this dumb "foggy window" shit doesn't count:
https://www.youtube.com/watch?v=0pE1qRAKoqA