My child isn't speaking yet. When they want something, they shout loudly.

It's a state of "I want it now — but the sound comes out before the words can." Rather than scolding: (1) first, respond in a visible way that "your request came through" (confirm with a photo, the real object, or a point: "this one, right?"), (2) also convey visually whether you "can" or "can't respond right now," and (3) have the family regularly use gestures and onomatopoeia (slowly, briefly) to expand the means of expression your child can use. Often, once they can speak well enough that words come out automatically, the need to shout disappears on its own.