Trying to create my first US Amazon ads: get errors that don't make sense

I've been selling on Amazon for over a year and advertising in Spain, Germany, France, Italy & UK. For some reason I am unable to create United States ads (although it appears in my Amazon Ads accounts overview) and I get strange messages when trying to create them...

First it says, when I attempt to create a campaign: There's an issue with your payment method. You can save your campaign as a draft now and update your payment method to launch your campaign. Go to Billing and payments. (My payment method is fine and working for my other countries.)

Then when I go to Billing & Payments it says: Your campaigns can't be launched based on the products selected. To advertise your products and launch campaigns, go to your Amazon Merch on Demand account and navigate to the advertise tab in United States. Learn about promoting your merch.

How can I do this if it won't let me select products? Do I need special permission to have access to advertise in the US, where I live? I really want to run ads for the upcoming holidays.