Teleportation

Back in the day, before proper, modern FTL travel had been invented, people were testing all sorts of methods, to see whether they worked.
They didn't, at least not properly. Teleportation, you know, classic teleportation. Breaking stuff down into atoms, transporting those, and then reassembling them. There were tries. Plenty, even.

History

It's been theorized for a while, but just wasn't good. But they did crack it. Humans, us, did get proper teleportation down. For the most part. Experiments began properly on Phobos, the small moon-ish body orbiting Mars, in 2311, just in case of any... unforeseen effects. Wouldn't want the halfway-terraformed surface or a cluster of habitats to be in danger, but a rock? That's fair game.
They did actually get decently far in the experiments, until the modern form of FTL travel rolled around, the Alcubierre-style drive sustem utilizing FTL fuel. Then experiments were called off, and resources driven to refining that. Nowadays, it's effectively lost technology, though not out of any strange cause, but rather just uselessness. It also had a myriad of issues, which really precluded widespread use.

Issues

A lot. Mainly the power draw. Fortunately, the power draw was roughly the same regardless of distance, but it was far too much. Another issue was how both sides of the travel distance had to have extensive preparation and machinery on them to allow teleportation. They did manage to teleport stuff from Phobos to Deimos, and even to Mars itself, though that took a massive amount of energy, the kind that would allow an entire warfleet to travel to another star. Ten times over. Safe to say, it just isn't practical.
There's also the issue that human memories and thoughts, everything that makes a person themself, wouldn't transfer. That's because of how the human brain works, those memories are stored as electrical impulses, and the teleporters couldn't work with that. Other stuff would be fine, though, and traditional AIs, on proper computers, could pass through.
Ironically, the Precursors do have teleportation, along with other stuff, so this tech is possible, and practical. Just not now.