Amazon message requires French social security number

Question

I’m an Amazon UK and France seller of foods. Over the past weeks our Amazon seller central has been issuing a message saying that sellers need to register for some sort of tax or social security thing. The document is in French so can’t understand it.
Could you let me know where I find the forms to register for French taxation? Not VAT as my threshold is too low (under 5,000 for France) and I am UK Vat registered. But the message was more to do with just declaring sales there for some other taxation registration. Can you help please?
Many thanks
Sandra