Advertising + VAT exempt

Hello
I’m a new UK seller and as I just started the business I am VAT exempt.
I noticed that on advertising invoices I am charged tax and emailed them to let them know I’m VAT exempt.

I got this reply:
Kindly know that in order to get VAT exemptions you need to register for VAT and add the VAT ID in your seller central account.
In this case, as per our records your account has no VAT ID added which is why the tax is being deducted.
If no VAT ID is provided on your account, the Tax charges will continue.

Can anyone guide me about this?
What is VAT ID and how should I have one if I’m VAT exempt?

Thank you