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 c35.c09224bb.usw1.send.mailgun.net is not the root of its zone.
Instead, try zone transfer on c09224bb.usw1.send.mailgun.net .
Instead, try zone transfer on c09224bb.usw1.send.mailgun.net .