If none of given solutions fixed the error, the issue probably about Google Cloud Billing settings. You must enable Billing on the Google Cloud Project at billing/enable.
{
"error_message" : "You must enable Billing on the Google Cloud Project at https://console.cloud.google.com/project/_/billing/enable Learn more at https://developers.google.com/maps/gmp-get-started",
"results" : [],
"status" : "REQUEST_DENIED"
}