I am sorry to hear you are experiencing an issue while adding your domain names to your Cloudflare account.
May I ask what error do you get when you try to add your domain names to your Cloudflare account?
Can you share your domain names so we could also check?
In case if you got "domain.com is not a registered domain" → Cloudflare 1049 error, may I suggest you to try looking into below articles to troubleshoot the issue:
May I ask if you did set the domain nameservers to point to a given set of Cloudflare nameservers (from your CF account → as for example if you know them while having other domains under the same Cloudflare account, so you made “one step ahead”) before you have added your domain name to your Cloudflare account (which is resulting the error you are experiencing “is not registered domain”)?
If yes, make sure to switch them back to your default’s nameservers and leave it to be like that for the next 24 hours.
Your domain should resolve normally (A records, NS records) before adding it to Cloudflare.
Furthermore, it might be an issue with the DNSSEC? If you recently changed your domain nameservers did you checked if the DNSSEC was disabled and any DS records removed at your domain registrar before domain nameservers were changed?
If so, kindly I’d suggest you to contact and ask your domain registrar to disable DNSSEC for your domain and remove any of the existing DS records at their interface.
Nevertheless, you might have to wait up to 48 or 72 hours for proper DNS propagation and to clear the DS/DNSSEC entries for your domain name.
Unfortunately, this is a know “issue”, or rather to say it happens. Before we change domain nameservers, we should make sure we disable the DNSSEC feature and remove any of the existing DS type of DNS records for our domain at our domainr registrar.