Gertrude Stein's comment on Ernest Hemingway's story refers both to the quality of the story and to its content. With the word "inaccrochable," Stein is using a French word as if it were English. She is also drawing on her strong interest in art exhibitions. The French verb "accrocher" means to hang something, as from a hook.

Stein is telling Hemingway that she thought the story was not good enough to benefit his reputation, which was important to him at the time, as he was not yet well known. Equally as important, she is referring to the story's sexual content. "Inaccrochable" was used in French to refer to erotic works that might even be considered pornographic and could not be displayed in a gallery. They might, however, be sold behind the scenes. Private collectors or connoissieurs might appreciate them and even show them to their friends, but they would not display them publicly.

Stein is reminding Hemingway that publishers are conservative about sexual content because of the controversies that were going on at that time. "Up in Michigan" was written in 1921, the same year that Joyce's Ulysses was the subject of an obscenity trial following its US publication; the verdict resulted in the book being banned.

In Chapter 2 of Hemingway's A Moveable Feast, he recounts Gertrude Stein's telling him that one of his stories was inaccrochable and then using a simile:  "That means it is like a picture that a painter paints and then he cannot hang it when he has a show and nobody will buy it because they cannot hang it either." (p.25, Restored Edition)

Now, what is the meaning of the simile?  Whether or not the work is good, no one wants to see it.  Ms. Stein actually told Hemingway that she thought the story, "Up in Michigan," was a good one.  However, it was not something she enjoyed reading and believed that others would not want to read it.