curl --request POST \
  --url https://api.pocketflows.com/campaigns/{id}/check_sendability \
  --header 'Authorization: Basic <encoded-value>'

{
  "sendable": true,
  "errors": {
    "business_missing_variables": [
      "<string>"
    ],
    "campaign_missing_variables": [
      "<string>"
    ],
    "customers_missing_variables": [
      {
        "id": "<string>",
        "missing_variables": [
          "<string>"
        ]
      }
    ],
    "unknown_variables": [
      "<string>"
    ]
  }
}