Seems too much of an interpretation, also there shouldn't be an article before 'mettle'.
I put 'this is a person without personality' (wrong, of course, all my answers are wrong these days) - does człowiek have to be a man? Personality is one of the options in the drop-down menu
I believe that "person" is fine.
But IMHO "personality" does not work here. The word "character" is used in this phrase exactly in meaning "4. moral strength, a combination of qualities such as courage, loyalty, and honesty that are admired and regarded as valuable". It is the Polish word "charakter" in meaning 1.b. , and "być osobą/człowiekiem bez charakteru" is a set phrase in Polish. It is similar in some way to the English "personality" in meaning "3. the qualities of character that make someone interesting or enjoyable to be with" - but it is not the same.
So, although in many meanings the English word "character" may be treated as synonym of "personality" translated into Polish as "osobowość" - it is not the case here. Someone's traits of personality can be better or worse, and they define this person's character. But I believe, that in this phrase - it does not work.
I have the same question. Does człowiek have to be a man? Can't be a woman??
Yes, it can be a woman. Although it requires a context in which it is clear, eg. a sleepy woman who is being disturbed by her children may say "Czy człowiek się nawet wyspać nie może?" (Can't a person get even a little sleep?)