I'm trying to figure out why my messages don't popup and I noticed in my settings that it says "category deleted" and when I click on In-app notification settings it says the category was preview new messages. How do I get this back?
Have you tried resetting your Settings?
Follow @SamsungRei suggestion first that should reset it to the default settings.
In the Samsung messages app settings/ notifications. When we tap on in app notification settings it opens a new page with the option to turn on or off preview new messages. The deleted category is usually related to a conversation that was set up for a custom notifications tone. If we delete that conversation then it marks it as a deleted category. Turn on preview new messages and make sure that the messages app default notification style is set to sound and pop-up.