Why is the field "Default country for VAT exclusive price" not showing any drop-down entry (am UK VAT-registered business)

Hello, hope you can help me - I'm a novice individual seller and have been required to produce a VAT invoice (UK seller and UK business buyer). I have provided the VAT registration number of my company whom I work providing consulting services from in order to do this. I sell < 10 goods on Amazon per year.

When I go to https://sellercentral-europe.amazon.com/tax/consolidatedRegistrations?context=enrollment, I see that my VAT registration number beginning "GB" is rejected. I get asked to provide a Business Registration Number & Citizen ID, though I've tried adding my company registration and passport number onto these without success.

I note that the field "Default country for VAT exclusive price" has nothing selected and no options in the drop-down menu.

I was asked by Amazon to update my business information on https://sellercentral.amazon.co.uk/sw/SSR/KYCInfo/spaDashboard - however this just asked to me to confirm my personal details and now my account features are currently suspended pending verification of my account which Amazon verified successfully back in August 2023.

Amazon support have so far been very unhelpful in this process and I'm worried I'm being legally required to produce a VAT invoice, but I have no way of issuing one or registering for Amazon's automated VAT calculation service.

I've tried to go through: https://sellercentral-europe.amazon.com/gc/amazon-business/tax-settings?ref_=sc_b2b_step_taxsettings - but am confused by this

And also to declare VAT exemption on: https://sellercentral-europe.amazon.com/tax/exemption/declaration - but I am told I'm not eligible for the exemption

Anyone with similar experiences or advice on what I should do?

Many thanks in advance for any help you can provide.