The error message is clear: you can’t add an A record when a CNAME record exists for the same belongly.com hostname. This is a requirement of the DNS standard.
You need to decide whether you’re keeping the existing CNAME record, OR you’re deleting it so you can add the new A record.
(Note that the CNAME record at the apex domain will be flattened to comply with the relevant RFCs, so you wont see this CNAME record when you do a lookup from the outside. But it should be in there on your Cloudflare DNS page.)