MANDATORY_NOT_FOUND error when trying to insert data

MANDATORY_NOT_FOUND error when trying to insert data

Hi, I am trying to use the CRM api to insert a lead record.
I have checked that the mandatory parameters are 'Company' and 'Last_Name' (as per the fields on the CRM page)

I still get the error code : MANDATORY_NOT_FOUND, api_name: data, message: "required field not found", status: error.

Here's my dummy json
{
    "Mobile":  "5555555555",
    "LeadOwner":  "leadOwnerleadOwner",
    "Salutation":  "Mr.",
    "Full_Name":  "firstName customLastName",
    "First_Name":  "firstName",
    "Company":  "companyName",
    "Phone":  "5555555555",
    "Country":  "US",
    "Last_Name":  "customLastName",
    "Email":  "email@example.com"
}