The payroll must have a completed preview (indicated by a preview.status of succeeded) before it can be approved.

{
  ...,
  "preview": {
    "status": "succeeded",
    "started_at": "2019-06-29T18:26:56.848920Z"
  },
  ...
}

Approving a preview approves the most recent preview of the payroll and moves the payroll status to pending, and the payroll must be reopened before it can be further modified.