Edit a cell Google Sheet API

Again I come with a problem with the Google Sheet API:

For this case, I have a spreadsheet or Spreadsheet and I want to modify / edit one of its cells. The request would look like this:

{updateCells={
    fields=*, 
    range={
        endColumnIndex=6, 
        endRowIndex=2, 
        sheetId=123456789, 
        startColumnIndex=5, 
        startRowIndex=1
    }, 
    rows=[{
        values=[{
            userEnteredValue={
                stringValue==D2*(1-Ficha!L6)+Ficha!L2*Ficha!L6
            }
        }]
    }], 
    start={
        columnIndex=5, 
        rowIndex=1, 
        sheetId=123456789
    }
}}

If I launch it, Google gives me the following error:

{
  "code" : 400,
  "errors" : [ {
    "domain" : "global",
    "message" : "Invalid value at 'requests[0].update_cells' (oneof), oneof field 'area' is already set. Cannot set 'start'",
    "reason" : "badRequest"
  } ],
  "message" : "Invalid value at 'requests[0].update_cells' (oneof), oneof field 'area' is already set. Cannot set 'start'",
  "status" : "INVALID_ARGUMENT"
}

I sense that the request is malformed, but I can not find the error

If someone could give me a hand ...

Thanks in advance

Greetings

PS: I'm doing it with Java, but seeing that the Google API encapsulates the requests I have chosen not to put it