Amazon Brand? Confused

I have a brand I have been working on for a few weeks now. I am in the middle of getting everything sorted out to start my Amazon brand registry, however, I am having a few doubts.

Even if I start my own brand, how do I know I will be able to sell on Amazon? I’ve noticed that some categories are “not accepting applications to sell” when I try to do a generic brand product. Super confused and could use some help, thanks.