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 v50.v5fc25cd0.use4.send.mailgun.net is not the root of its zone.
Instead, try zone transfer on v5fc25cd0.use4.send.mailgun.net .
Instead, try zone transfer on v5fc25cd0.use4.send.mailgun.net .