Diamond ore generation peaks at y ≈ -58 in Minecraft 1.18+. The pre-1.18 advice of y=11 is obsolete.

Below y=0, the ore is almost always deepslate_diamond_ore, not diamond_ore — search for both names.

An iron_pickaxe is required — diamond ore mined with a stone_pickaxe or weaker drops nothing. Craft the iron_pickaxe (3 iron_ingot + 2 sticks at a placed crafting_table) before mining diamond. A diamond_pickaxe is not required.

To descend quickly, call go_near with your current x/z and a deep y (e.g. {"pos": {"x": <x>, "y": -55, "z": <z>}}) — the pathfinder digs straight down in one action. Don't mine_block stone repeatedly to descend; it wastes turns and pickaxe durability.