Sorry that you are having issues. Hopefully we can get you all taken care of.
Some questions to help troubleshoot:
1) Are you using the built-in functionality? E.g. did you unlock the DIGITS functionality and then setup the lines in settings > accounts > multi-line settings?
2) If you did, then when you look at the DIGITS line you are trying to use, does it say "Ready to Use" or does it say something else?
3) If you're using the app instead (I'd recommend using the native functionality...) though you can check to see if the line shows as available there if that's what you're using
Getting those out of the way and assuming that the line is in the "ready to use" state...
A) What kind of network were you on?
- If it was WiFi is WiFi calling enabled? If so are you able to send messages from the native line?
- If it was cellular what was the cellular indicator showing? 4G LTE, 2G, something else?
B) Are you able to send any messages or is it just this one particular message or messages when you're in one particular place
Finally are you on the latest software update for the Note 8. If not, can you update and try again?
Once we get all these preliminaries out of the way and if you're still having issues, we'll get some more detailed info if needed.