When you buy a domain using Cloudflare Registrar, Cloudflare sets the nameservers and they can’t be changed, as is indicated at the time of purchase.
If by that you mean you added NS records for the apex domain in your Cloudflare DNS that won’t work. You will need to point your domain at your host using A/AAAA/CNAME records.
I had successfully configured redirections on my previous domains, but I wasn’t aware of this rule. I think I’ll stop using these services to avoid facing the same issue with other domains.