Have you downloaded anything recently such as an app etc that could have affected this @ColXsara
Go into Settings, Notifications, Brief Pop Up Settings and it should be there.
i.e
If it's not then Try putting the phone in Safe Mode which disables everything apart from the stock apps.
Put the phone in Safe Mode which disables your downloaded app's but let's your stock app's keep working. If the issues stop then this highlights a downloaded app as the possible issue. Come out of Safe Mode and start deleting your most recent downloaded app's one at a time until it stops. https://www.samsung.com/uk/support/mobile-devices/how-to-start-my-galaxy-device-in-safe-mode/
