Some magazines and journals accept unsolicited submission of novel excerpts, but most do not. Always read writers’ guidelines to see what individual publications look for in submissions.
Some writers find that a portion of their novel can stand alone as a complete short story and submit that. Technically, it is an excerpt, but it also functions as a complete work and fits the bill of a short story. Journals and magazines are open to this practice. And it can be a good way to build anticipation for your novel. But keep in mind: The story has to be its own entity. It cannot rely on other parts of the novel in order to make sense or feel satisfying and complete.
Flip to the copyright page of some novels and you’ll find acknowledgement of those previous publications. Will Alison published several portions of his novel, What You Have Left, as short stories. Dean Bakopoulos’ short story “Please Don’t Come Back From the Moon” is the first chapter of his novel of the same title.
At most magazines and journals, copyright reverts back to the author upon publication. Make sure that’s the case if you get an acceptance for this kind of short story. Be aware of what rights you’re giving up, so that you don’t give up the right to publish that section as part of your novel.