1)When you have logged on first or two times then you can see the options for adding categories. Now what you can do is create a ticket in seller central and asks Amazon to unlock categories.
There are two types of categories on Amazon. Lock category and Open category. For lock or restricted category ask Amazon to unlock it and sometimes they ask for performa invoice or sometimes they open it without PI.
Some of the lock categories include cosmetics.
Other open categories are home and kitchen, toys category etc.
2)Please guide me in the process of registering new product categories as well.
>>>To add categories under GST you have to contact with local CA about it.
3) I’m unable to figure out the option using which I can download my GST Certificate.
>>>First you can talk to your CA. You can log into GST portal -> go to services ->Now find view/download ->click.
Let me know if you need any help.