What does excuses are tools of incompetence used to build monuments of nothingness mean?

It means excuses are things used by people who are incapable of doing things. And they keep using them/ building them on top of each other, making monuments. Monuments of nothingness, because excuses mean nothing. So those who specialize in excuses are seldom masters of anything. Meaning they'll never master or be good at anything as long as insist on using excuses.