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 v5129.v54f91261.use4.send.mailgun.net is not the root of its zone.
Instead, try zone transfer on v54f91261.use4.send.mailgun.net .
Instead, try zone transfer on v54f91261.use4.send.mailgun.net .