You can directly manage the inventory for all kits currently on sale in the Creator Center.
How to Manage Kit Inventory
Creator Center > Inventory > All Inventory
You can set quantities or mark items as out of stock through the 'Manual Setting' feature in the All Inventory menu.
Important Notes for Out of Stock Processing
When you need to process an item as out of stock, please set the inventory to -999 instead of 0. Setting it to 0 may cause inventory to reappear due to returns or other reasons.