Amazon has restricted some sellers from selling in various categories (e.g. Grocery, Health and Beauty, Clothing, Watches, Shoes, etc.)
I believe Amazon made those restrictions because the categories were getting flooded with fly-by-night sellers. Now Amazon requires an application process - which can be rather lengthy for some categories - that can help reduce (or eliminate) their number of illegitimate sellers.
Everyone who wants to sell in those categories, naturally, has to apply for approval (or 'ungating').
The first thing is: don't panic.
Second (Free): Read EACH of these blog posts from a seasoned selling expert (much smarter than me) (direct links, no opt-in required):
1.) Find out what categories you're approved/not approved to sell in, here
My additional advice: if you are restricted from a category you were/want to sell in AND you can't get approved in that category, there are still opportunities in the 'default' categories you are not restricted from (used/new: Toys and Games (Amazon calls most used ones 'collectible'), books, CDs, Tools & Home improvement, Baby, Electronics, Kitchen & Household, etc.)
Good luck, and keep me posted!