could not run legacy post zone sub request against new zone products: failed to update user subscription: failed to apply object products: You cannot add or modify subscriptions or services until the outstanding balance is paid. You should be able to do so in your Billing page.
Indeed, I get the same error when I try to add this specific domain. But I’m able to add other domains just fine. That means the problem is with this specific domain, and not the entire account.
This domain was previously active on Cloudflare and even still has the Cloudflare nameservers. So it’s likely there are some unpaid bills for this domain in your account OR the account that the domain was previously active in (if different from your current account).
Please open a Billing ticket for the Cloudflare support team to look into this for you. The community cannot, unfortunately, help with billing and account issues: https://dash.cloudflare.com/?to=/:account/support