Amazon Error 4400: Your seller account is not approved to offer seller-fulfilled products in this category at this time
If you are receiving an error 4400 saying you need authorization for a specific category, then this is generally something you need to talk to Amazon about.
Some categories require you to be approved by Amazon before you can sell in, and sometimes certain categories can be temporarily restricted (gated), for example during the holidays - as the following quote from Amazon seller support shows:
"The 4400 error is a category specific error, more specifically for Toys & Games. The error was stating that you were not able to list seller-fulfilled listings within this category because this category has the Holiday Gate restrictions on it."