freak4pc (JIRA User) this happens if you run it in docker and your container isn't privileged. Swift's REPL needs a privileged container because it uses ptrace to attach to another process.
if you do

docker run -it --rm --privileged swift:5.0

it should work. However, please be aware that this has some security implications: https://docs.docker.com/engine/reference/run/#runtime-privilege-and-linux-capabilities

In local development the security implications should be okay though 🙂

Comment by Shai Mishali (JIRA)

Thanks Johannes!
That seems to do the trick. Maybe a better error message is in order?

@dcci, how hard do you think it would be to detect this situation?

I think it's feasible. Let me see what I can do.