question

I would make sure you have associated the modifierGroup with the Item. You can do so through the Merchant Dashboard on the item creation and edit item pages or the Rest API.

Endpoint:
POST v3/merchants/{mId}/item_modifier_groups

Example Request Body:

{
                
     "elements": [
          {
               "item": { "id": "*************" },
               "modifierGroup": { "id": "*************" }
          }
     ]
}

Here is a related thread where I breakdown the process of setting up modifiers for an item:
https://community.clover.com/questions/14181/clover-inventory-api-to-load-category-items-only-w.html