From what I read I gather that you’re probably not VAT registered. VAT registration is something you do with the financial / tax authorities in your country. It’s not something that Envato does for you or that automatically comes with being an envato author.
Edit: Do not confuse this with the fact that Envato collects VAT (in some cases) when you make a sale. They handle everything VAT-related on behalf of the authors but that has nothing to do with being VAT registered.
I can only speak for Germany, but I guess it’s the same anywhere in Europe: If you’re VAT registered you’re obligated to collect VAT by adding the proper amount (in Germany normally 19% at the moment) to any invoice you give out to clients (eg. for custom jobs). When you receive the full amount from your client, the VAT does not ‘belong’ to you since you have to remit it to the tax authorities. The great thing about VAT registration is that you’re also allowed to do the opposite - you’ll receive back any VAT that you paid on invoices related to your business.
So - if you’re not VAT registered with the tax authorities in your country, you’ll just have to tell this to PRS when joining. No certificate needed.