eBay API {sku} could not be found or is not available in the system

According to documentation createOrReplaceProductCompatibility

I've added as required but for some reason I'm getting response

{
    "errors": [
        {
            "errorId": 25702,
            "domain": "API_INVENTORY",
            "subdomain": "Selling",
            "category": "REQUEST",
            "message": "15955 could not be found or is not available in the system",
            "parameters": [
                {
                    "name": "text1",
                    "value": "15955"
                }
            ]
        }
    ]
}

What I'm doing is going with request to

PUT https://api.ebay.com/sell/inventory/v1/inventory_item/15955/product_compatibility

And my headers contains

Authorization:Bearer ....
Content-Language:en-US

Body:

{
    "compatibleProducts": [
        {
            "productFamilyProperties": {
                "Year": "2019",
                "Make": "Chevrolet",
                "Model": "Blazer",
                "Trim": "Base Sport Utility 4-Door",
                "Engine": "2.5L 2457CC 153Cu. In. l4 GAS DOHC Naturally Aspirated"
            }
        }
    ] 
}

POSTMAN:

This looks to me as you are using regular items instead of inventory items.

First go with migrating-listings. This will switch your items to inventory items, and then you will be able to find the items over SKU.

One more way to check is to go with :

GET https://api.ebay.com/sell/inventory/v1/inventory_item?limit=100&offset=1000

It should return you

{
    "total": 0,
    "size": 0
}