Go to your Tax Settings here or navigate to Settings > Tax Settings > Add VAT Numbers
You can "Edit" the current VAT Registration Number or "Add a VAT/GST registration number" by clicking the link at the bottom.
You will need to select the country, add the VAT/GST Registration Number and the Registration Number Address to complete the process.
You can read Registering for VAT – Amazon VAT Services - Amazon.co.uk for more details.
Go to the VAT calculation service here or navigate to Settings > Account Info > VAT calculation settings (under tax information)
Accept the terms of conditions
Confirm that you are applying the correct VAT rate (for most businesses this would be A_GEN_STANDARD)
Set the start date for VCS. The earliest date is often 1-2 weeks in the future.
All EU sellers who sell above certain thresholds in the country where they are established must register for VAT.
If you use FBA and store any units (even just one) in another country, you are liable for VAT in that country.
Amazon will block your account if they believe you are required to register for VAT and you have not provided your VAT numbers.