"I was missing that."
Translation:Tio mankis al mi.
Just to clarify: the use of "missing" is rather troublesome here since missing in English also signifies (e.g. accidental) loss.
Most people will look at this sentence and see "I'd lost that." But in reality the translation desired is that for "I lacked that," or, even more tortuously "That was lacking for/to me," since manki is intransitive.
Perdi is the word for loss, manki is the word for lack. But perdi is transitive and manki is not. So for remembering manki one should think of a few very idiosyncratic uses of lacking in English such as "The service here is lacking," or "The rich never lack for company" (actually even this one is arguable depending on which direction you think the preposition is going). Obviously these sound old fashioned and a bit odd, which is no doubt why the course creators chose the more ambiguous missing.
For those who believe that the characteristic of transitivity is somehow self-evident or "obvious" from the meaning of the word, let this serve as a counter example.
Could you expand on that? From here, http://literaturo.org/HARLOW-Don/Esperanto/rules.html, it looks like the dative case in Esperanto is formed by a preposition, the "al mi" in the official correct answer.