Can't upgrade from Mountaion Lion to Mojave: Please verify billing to update. Please update to verify billing.

Hi.

I can't upgrade from Mountain Lion to Mojave through the App Store.

When I try, it says I have to verify my billing information because it's the first time I use the App Store. When I press "Verify Billing", it says I can't verify my billing information because I have to update my macOS to Mojave. So I go through the website and ensure everything is set up correctly, but when I go back to the App Store it completely ignores this and wants me to verify again.

How can I get past this to upgrade my macOS?

Thanks.