https://{tenantName}-admin.sharepoint.com/_api/search/query?querytext='contentclass:sts_site'

500 Internal server error
{
  "error": {
    "code": "-1, Microsoft.Office.Server.Search.REST.SearchServiceException",
    "message": {
        "lang": "en-US",
        "value": "An unknown error occurred."
    }
  }
}