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 _15b51be8cfd30c4a85ec4586793f47d0.onlinemail.io is not the root of its zone.
Instead, try zone transfer on sectigo.com .
Instead, try zone transfer on sectigo.com .