This one doesn't make sense.
Completely agree with all of these comments.
I think "Acabei de fazer o meu trabalho" should be translated as "I just did my job"
It seems they meant to say " I´m just doing my job"
How does this test the infinitive?
Fazer can mean "to do" or "to make" depending on the context. So in this case the translation should be "I just do my job" of "I just do my work" I'd say.
Not only that, but "acabei de" implies that the action end just a while ago, so in English you should use the present perfect.
Argh, duolingo, you got this one way wrong. An English version of "Eu só faço o meu trabalho" would be "I only do my work." Meaning, I don't do anyone else's tasks, only mine. "I just make my job" is nonsense.