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 v599.v576a36ee.use4.send.mailgun.net is not the root of its zone.
Instead, try zone transfer on v576a36ee.use4.send.mailgun.net .
Instead, try zone transfer on v576a36ee.use4.send.mailgun.net .