place-sand.js

const bot = mineflayer.createBot({ /* ... */ });

// ... do some action that would get you within range of a piston ...

// This .findBlock method will find the nearest block to the point, which is the position of the bot's entity. The block it finds must match the piston block ID, and will be returned.
const piston = bot.findBlock({ point: bot.entity.position, matching: pistonBlockId });  

if (piston == null) {
  return;
}

// the null is required for some reason, but doesn't do anything
const sand = bot.inventory.findInventoryItem(sandBlockId, null);

if (sand == null) {
  return;
}

const onSandPlace = (error) => // ...

const onSandEquip = (error) => {
  if (error) {
    return;
  }
  
  // this vec should probably be different based on the face you want to place it on
  bot.placeBlock(piston, new Vec3(0, 1, 0), onSandPlace);
}

bot.equip(sand, 'hand', onSandEquip);