We in the states mostly always use the term employee when speaking of a person who works for you. The term is used in legalese contracts etc.
Why is it implied that the worker is talking to their/some other particular boss? In other words, why 'a boss' does not fit?
Well... it seems rather strange to me, "a worker" is a profession, but "a boss" is not exactly a profession, I don't think it makes sense to talk with "someone who is a boss somewhere"...
Why is a translation "An employee is toking to the boss" graded as incorrect? (When I use "employee" for "robotnik" in other phrases/sentences it is accepted).
I don't see "employee" being accepted in other sentences about "robotnik"... and it's too general, too vague, after all "robotnik" does manual work.
An employee can also do manual work. An employee is anyone who is employed. I did not know that robotnik is so specific. So laborer would/should also be accepted