Expanding a little on what @Habsfan had mentioned(Thank you for answering:)), Rich Communication Services(RCS) is a new implementation of a messaging system replacing SMS.
Here's an article detailing what it might look like for you and your Samsung Note8:
As that seems to be a connection warning, it might help to double check your connection settings, and make sure both your Wi-Fi and cellular connection are solid.
Unfortunately in the rollout of RCS not all the carriers are included quite yet. Theres an always growing list of carriers so in the future it might be able to work.
I hope this helps!