This read-only DisplayString holds the human-readable error text from the most recent TEK Invalid message this TEK service received, returning an empty string if none has been received since the service started. It provides descriptive context to pair with the numeric cwrBpiRsTEKInvalidErrorCode when troubleshooting a rejected key request. For example, an admin diagnosing repeated TEK rejections on a subscriber radio would read cwrBpiRsTEKInvalidErrorString for plain-language detail beyond the bare numeric code.