Remco Ah, you're probably trying to set up the account you're calling "Main" as the account that is receiving the VAT payments. Then it makes sense that you can only select all the other sub-accounts you have.
Here you can see how to set up AutoVAT: https://together.bunq.com/d/14717 You have to create the sub-account first where you want to receive the payments, then set it up from there.