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From the menu: Select - Modify - Contract does not work. The selection does not contract no matter how many pixels you specify.
What version of Photoshop and macOS are you using?
Can you post a screenshot of Photoshop including your selection?
It will always stick to the canvas edge, if that's what you mean. This is by design. It only contracts from "free-floating" pixels.
If indeed you have a selection that touches the document bounds on one or more sides. then in the Select>Modify>Contract dialog check Apply effect at canvas bounds.