You seemingly have set valid nameservers and you will have to wait for propagation now. That can take up to two days, but I’d try it again in a couple of hours.
that’s why I’ve shared the dnschecker link. it already looks well propaged.
Cloudflare doesn’t mention I’ve bad NS, it says my domain isn’t registered. you don’t need to configure NS when you want to check if a domain is registered. That’s why I’m surprised
This is precisely why you don’t add nameservers that you haven’t been explicitly given for that domain - they are one of the steps in authenticating which account is adding the domain.
If I add your domain to my account & you add it to yours, we’ll have different nameservers in the setup wizard and whichever ones get added determines whose account it is added to.