default-containers:

[YouSeeMeRunning2]

Superiorskyblock latest developer version Yatopia 1.16.5 (Fork of paper) https://pastebin.com/2LHgx9tW (Config)

When loading up a new island the chest doesn't have anything in it

[OmerBenGera]

The default containers section is the following one:

# Set items for containers inside island schematics.
default-containers:
  # Should this setting be enabled?
  enabled: false
  containers:
    # The type of the container.
    chest:
      # The contents of the container.
      '0':
        type: ICE
        amount: 2
      '1':
        type: MELON
      '2':
        type: TORCH
        amount: 2
      '3':
        type: BONE
      '4':
        type: LAVA_BUCKET
      '5':
        type: PUMPKIN_SEEDS
      '6':
        type: SUGAR_CANE
      '7':
        type: RED_MUSHROOM
      '8':
        type: BROWN_MUSHROOM
      '9':
        type: CACTUS
      '10':
        type: BREAD
      '11':
        type: WHEAT
      '12':
        type: LEATHER_BOOTS
      '13':
        type: LEATHER_LEGGINGS
      '14':
        type: LEATHER_CHESTPLATE
      '15':
        type: LEATHER_HELMET

As you can see, yours is a bit different - you need to configure which container is used. In the default example, it's a chest.