The red exclamation mark warning means that the Messages app failed to deliver the message. The issue can stem from different things depending on the type of message you're trying to send.

A light appeared on the dashboard of my 2012 Kia Sedona looking like a red enclosed exclamation point with a circle around it. I looked it up in the manual and there is no explanation. Nothing is ...

Your network is the main reason why you're seeing the red exclamation mark when sending an SMS through the Messages app. In this case, you can try to fix the network issue by turning Airplane Mode ...