In "Death of a Salesman", what does Willy misunderstand concerning the sales profession and the business world in general?
Willy references the future "business world" ability/savvy of both of his sons as being far ahead of their current classmates because his sons, as he states, are like "Adonis".
The mythological allusion to Adonis is of a very handsome, but foolish, young man who believes that his beauty and youthfulness will protect him, yet he perishes in a vile animal attack.
The reference by Willy is ironic - he claims that his sons do not need to worry of ample school learning: They are so attractive, thinks Willy, that the boys will automatically succeed. As seen in the allusion, this is not so.
Willy is demonstrating that his inability to make it in the sales/business world is due to his inability to adapt to a changing world. He is superficial, not skilled.
First, Willy doesn't understand that in a more modern world, a closed deal takes more than a man's word and a handshake. Since Willy lives in the past, he cannot understand how he can be fired after his many years of service to his company, and that his personal problems have no effect on a younger generation of management. Business is impersonal and is focussed only on the bottom line.
Willy's advice to Biff about finding a job is as useless as Willy's old-fashioned ideas about business. A person needs skill and identification with the product in order to be considered for employment. Being "well liked" has nothing to do with success.