DNSKEY flags only supports 0-255

DNSKEY record 256 is the public key called Zone-signing-key, used to verify the DNS record signatures for A, MX, CNAME, SRV, etc.
DNSKEY record 257 is called the Key-Signing Key, used to verify the signatures of the DNSKEY, CDS, and CDNSKEY records.

But cloudflare DNSKEY DNSKEY only supports 0-255.

Cloudflare offers DNSSEC with one click:

Moreover, you would need to add the given DS record from the Cloudflare to your domain registar.

There are differences between Zone-Signing Keys and Key-Signing Keys. That is why you cannot add 257 to your domain via Cloudflare dashboard, and keep in mind if you are using some of the older or unsupported algorithms rather than algorithm 13.

Please, find more info about it on the links below: