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.
What is the issue you’re encountering
I cannot set up new website. I’m the new owner of domain. It could be previously setup with Cloudflare. I’m not sure about that. I can add any other domains, except that one.
What steps have you taken to resolve the issue?
I checked my billing page - I don’t have outstanding balance.
Billing issues requre a Billing ticket with Support