When Gatsby says that he cannot make Daisy understand, what is it he wants her to undertsand?

Gatsby wants Daisy to understand emotion, something her upbringing has failed to instill in her as important, and something she is unable to feel.

"But the rest offended her — and inarguably, because it wasn’t a gesture but an emotion. She was appalled by West Egg, this unprecedented “place” that Broadway had begotten upon a Long Island fishing village — appalled by its raw vigor that chafed under the old euphemisms and by the too obtrusive fate that herded its inhabitants along a short-cut from nothing to nothing. She saw something awful in the very simplicity she failed to understand."

Further down in the chapter, Gatsby and Nick converse about the past and the fact that in Gatsby's heart, he wants Daisy to erase time and admit she never loved Tom. She can't do this because it isn't true, and what she cannot understand is that in reality, all he wants to do is hear it. 

“I feel far away from her,” he said. “It’s hard to make her understand.”

“You mean about the dance?”

“The dance?” He dismissed all the dances he had given with a snap of his fingers. “Old sport, the dance is unimportant.”

He wanted nothing less of Daisy than that she should go to Tom and say: “I never loved you.” After she had obliterated four years with that sentence they could decide upon the more practical measures to be taken. One of them was that, after she was free, they were to go back to Louisville and be married from her house — just as if it were five years ago.

“And she doesn’t understand,” he said. “She used to be able to understand. We’d sit for hours ——”