In Of Mice and Men, what message does Steinbeck convey to the reader about the American Dream, and how does he go about presenting this information through his characters?

In Of Mice and Men, Lennie and George dream about owning their own farm and living off of the land.
The American dream is the belief that everyone has an equal opportunity for success, provided that they work hard. Of Mice and Men is set during the era of the Great Depression when, despite extreme shortages, people still clung to the idea of the American dream. Ultimately, what Steinbeck demonstrates with Lennie’s death is that for some people, despite determination and drive, the American dream is unattainable and can even be somewhat of a trap. However, Lennie and George's dream is what keeps them together (see quote below) and striving for something greater. So, although the American dream is just a fantasy, perhaps it is necessary for living a fulfilling life.

Guys like us, that work on ranches, are the loneliest guys in the world. They got no family. They don't belong no place....With us it ain't like that. We got a future.... An' why? Because...because I got you to look after me, and you got me to look after you, and that's why.

Another event illustrating the futility of the American dream is Curley’s wife’s death. She revealed that her dream was to be a movie star, but she is unable to escape her unhappy marriage and life on the ranch. Her dream was unlikely to ever materialize, but Steinbeck absolutely crushes even the possibility by penning her death at the hands of Lennie.
Crooks also allows himself to become carried away with the thought of hoeing the garden on George and Lennie’s future farm, but the underlying sense is that this will never happen. Moreover, Crooks himself explicitly says that American dreams are impossible:

I seen hunderds of men come by on the road an’ on the ranches, with their bindles on their back an’ that same damn thing in their heads. Hunderds of them. They come, an’ they quit an’ go on; an’ every damn one of ‘em’s got a little piece of land in his head/ An’ never a God damn one of ‘em ever gets it. Just like heaven. Ever’body wants a little piece of lan’. I read plenty of books out here. Nobody never gets to heaven, and nobody gets no land. It’s just in their head. They’re all the time talkin’ about it, but it’s jus’ in their head.

Steinbeck conveys the message that the American Dream—which can be defined in the context of this book as people's ability to control their own land and destiny—is not available to the American working person. Steinbeck conveys this message about the inaccessibility of the American Dream through the fate of his main characters, Lennie and George. They dream of owning their own farm where they can raise crops and rabbits and are not subject to the management of cruel farm owners. This dream is not realized, though, because George mistakenly kills a woman, so Lennie must kill him to avoid the authorities further inflicting damage on George. Candy, an elderly farm worker, symbolizes the working man's inability to achieve the American Dream. He becomes entranced by the dream Lennie and George have of owning their own farm, but he also fails to achieve this dream. It is clear that the farmhands in the book will meet tragic ends in which they either die or face lives of unending work with little to show for it.