If you notice in your Samsung Galaxy Note 9 that in the lower left corner you see the text "Safe Mode", this means that you have booted your Samsung Galaxy Note 9 into a special mode.
More information about this mode can be found here. Normally, Safe Mode is terminated by restarting the phone.
If this doesn't help you, please check if the following is true for your Samsung Galaxy Note 9:
1. The protective cover jams the Volume button
2. The Volume button is depressed
3. The Volume button has a mechanical defect
We hope that there is no technical defect, but the Volume button was only jammed by your protective cover.
You now know why the Safe Mode on the Samsung Galaxy Note 9 cannot be terminated.