Knowing that your text messages are being sent properly and there aren't any errors in the Verizon Messages application is fundamental, especially with your five-day-old phone. I'm certain we can get to the bottom of this and avoid the confusion of this peculiar unsent notification your device is getting.
I have a few questions that will help me gain more insight with the issue at hand. Does this happen with multiple different recipients you're sending to? How often does this happen with messages that don't have media attached to them? Also, to clarify, did the unsent message come back even after you re-sent the message that had the error?