Sharepoint - A type named 'SP.Data.ClientsListItem' could not be resolved by the model. When a model is available, each type name must resolve to a valid type

Try it as below:

var item = {
        "__metadata": { "type": "SP.Data.CaseListItem" },
        "Title": "updated title"
    };


    $.ajax({
        url: _spPageContextInfo.webAbsoluteUrl + "/_api/web/lists/GetByTitle('CaseList')/items(1)",
        type: "POST",
        contentType: "application/json;odata=verbose",
        data: JSON.stringify(item),
        headers: {
            "Accept": "application/json;odata=verbose",
            "X-RequestDigest": $("#__REQUESTDIGEST").val(),
            "X-HTTP-Method": "MERGE",
            "If-Match": "*"
        },
        success: function (data) {
            console.log(data + " success in updating item");
        },
        error: function (data) {
            console.log(data);
        }
    });