Changes since SOA version
IXFR is a zone transfer request that returns changes since a specific SOA version. Enter the SOA version to request changes since then.
Not an apex domain
The domain cardiovascular-cockroach-j9u0gki8lq2hrptsugkbc5u6.herokudns.com is not the root of its zone.
Instead, try zone transfer on herokudns.com .
Instead, try zone transfer on herokudns.com .