Hello Habitat,
Welcome to the Forums.
Are you attempting to add the Tax ID number to a Personal, Premier or Business account? (You did state you where trying to set up a Merchant account) The reason I ask is because you can only add a Tax ID number to a Premier or Business account. You can view your PayPal account type by logging into your account, once you log in you should be on the My Account page, right under Welcome and your name you will see what account type you are. You will also want to make sure the name on the account is a persons name and not a business name, that will cause errors as well.
Let me know if this does not make sense.
~ Misty
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