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 recently bought a website from someone, and I tried to add this website to my Cloudflare account, but the error message (What is the issue you’re encountering) keeps appearing ) since this is the first time I have added the website to my account and my account has nothing pending
What steps have you taken to resolve the issue?
I tried to contact support, but nothing helped, they didn’t respond.