Here are a few simple steps you can try to get your issue resolved. SInce I don’t know the full history of your Note 8, any troubleshooting step might be useful in fixing the problem.
1. Perform a simple restart. By restarting your device and refreshing the system, this may address the bug that causes the SMS glitch.
2. Clear its cache and data. Sometimes temporary files or cache may get corrupted, resulting to app slow downs or worse. If wiping an app’s cache won’t fix the problem, make sure to follow it up with clearing its data.
3. Try sending SMS in "Safe Mode".
4. In some instances, you're able to fix texting issues by simply removing, then reinserting the SIM card. Be sure to turn the device off first before removing the SIM card. The same is true before you insert it back.
5. Try using another messaging app.
I hope this bit of info helps in any way. Have a great day.