• Your vendor is expecting to receive certain EDI segments in your EDI files. That's why the INCLUDE_EMPTY fields are available in EDI Attributes. You can't just opt out of sending those segments on a per-order basis, even if there's no value to be sent. I.E. If you normally send shelving location, but didn't assign shelving location on one item, you still have to send the blank value otherwise your whole order will fail.
• There are some segments that are only specific to a particular type of EDI message. For example, MOA+86 is the total invoice amount, so it's only present in invoice EDI messages.

Use of uninitialized value $c{"buyer_code"} in concatenation (.) or string at /usr/local/share/perl/5.22.1/OpenILS/Utils/EDIWriter.pm line 454.

You are missing a value in your EDI account in the Vendor Account field.

If using EDI attributes and using the BUYER_ID_ONLY_VENDCODE or BUYER_ID_INCLUDE_VENDCODE options, you might also be missing a value in the Vendor Code field of your EDI account.