You do own the base game, yes? That is required.
From the Origin page: To use this, you must have: The Sims™ 4
I'm having the same issue. When I attempt to buy the Moschino stuff pack I get the message...
'An error occurred. Please try again later. [ref. #Error: httpRequest ERROR: https://gateway.ea.com/proxy/commerce/carts2/store-cart-purchase-client/offerEntries?storeId=Store-O... 400 (Bad Request)]'
I have the base game and many other packs so this should not be an issue.
Can you check if the following info helps you with that 400 (Bad Request) error: