An API key must be used by a single device

(164) An API key must be used by a single devicereturned from:$xml = new SimpleXMLElement(LimeLM::GeneratePKeys($prod->api_key, 1, 1, $payer_email));

any idea how this error occured?

The docs contain the error codes with full descriptions: https://wyday.com/limelm/help/api/limelm.pkey.generate/#error-codes