What does it mean when my sent message turned green and did not indicate that it was sent?

The message being green indicates it is sent as an SMS cellular text message, and not an iMessage (which would appear blue).

SMS text messages have no capability for receipts, so not indicating "sent" should not be surprising.

most likely the recipient's phone is either off, inoperable, out of cell tower range, or battery is too low / dead at the time