When trying to add DKIM and SPF records to my domain and I’m getting the error “Warning: cPanel is unable to verify that this server is an authoritative nameserver for domain.com”

Warning: cPanel is unable to verify that this server is an authoritative nameserver

That message typically indicates the use of external name servers for the domain name associated with the account. So since the domain domain.com is using outside server nameservers you would actually just need to copy the DKIM and SPF records and add them in the DNS zone where DNS is hosted.