We become builders by building and we become just by doing what is just. In the light of this statement what cannot be asserted?

The claim that cannot be asserted is that virtue is a mechanical and non-ethical skill.

Saying we become just by doing just acts treats virtue as acquired through practice, like a skill.

So virtues resemble skills, and a person who forms them is a virtuous being.

A skill too is genuinely gained by repeatedly performing the relevant activity.

But the statement makes virtue an ethical excellence built by habit, never a mindless mechanical thing.