What dose ares mean when he says percy "doesn't have what it takes"? is this a fair criticism

In any case someone says someone else doesn't have what it takes, it usually is meant in a negative way, sometimes not however. By saying this, you are claiming that another person does not have the full potential or capability to successfully accomplish a certain task. This is what he most likely meant to Percy, however in other cases it may be meant in a way to be neutral, though the speaker may not realize it's negative effect. This is not fair or just criticsm, because he does not know about Percy's experiences or weaknesses, or let alone his strong potential. He judges without thinking and doesn't realize the talent and importance he holds as a character.