Translation:The happy cook likes his profession very much.
Can this also mean "the happy cook likes HER profession very much"? Is kuiristo only for male cooks, or can it mean a cook regardless of gender?
"The happy cook likes his own profession very much" Why is this wrong? I would have put "his profession" if it was "lian profesion." I thought it needs to be "his own" for "sia." What is the real difference between "sia" and "lia" (or between "sia" and "sxia")?
Li ŝatas lian profesion. = He (Ronald) likes his (Jack's) profession.
Li ŝatas sian profesion. = He (Ronald) likes his (own) profession.
"sia" can only refer to something belonging to the subject of the sentence.
English does not have a separate word for this and so "his profession" is ambiguous between "belonging to him (the subject)" and "belonging to him (someone else)".
So "sia profesio" and "lia profesio" would both be translated as "his profession".
If you want to say "his own profession", that would be "sian propran profesion", with "propra" for "own".
If "sian profesion" refers to his own profession, what would be a reason for using "sian propran profesion"? Would it be to specifically distinguish his own profession from others?
I used "The happy chef is really likes their job." and it was wrong for some reason.
It's an issue with your English grammar. I submitted "The happy cook likes their profession very much," or something like that, and it worked.
I tried "the happy cook really likes his/her profession", but it was not accepted.
Why is "love" not accepted for tre sxatas? In English it's frequently used for non-romantic meanings.
In English there is definitely a difference between like and love as in Esperanto. There's even a joke about it that I shall not repeat here.