Hasta la vista.. lukas
Isn't 'Until later Lukas' correct as well?
Yes, I think it should be. Make sure you report it the next time you see it so that it can be added as an alternative.
It's not incorrect, but it does not sound like something I would expect a native-speaker of English to say.
If I remember well, some beginning lessons in Duolingo were accepting "Until later", so this one should, at least to have consistency.
"Until later, Lukas" is fine English. It sounds a little formal, but I could imagine it as the sign off to an email, for example.
It would be nice if Duo found some native English speakers (from various places) to vet colloquial phrases. There are lots of ways to say this in English.