EML Create by association on existing Root

I am trying to create a child instance for an existing root using EML. My understanding is that %cid and %cid_ref are for working with the transactional buffer CREATE + CREATE BY ASSOCIATION). In my case the root already exists with a valid key. The root entity is Case with esch_case_key as the key. The child entity is Log with esch_case_key and esch_seq_no as keys. I am getting an "unspecified" error in the FAILED table. Can anyone see what I am missing below? Thanks in advance.

MODIFY ENTITIES OF zi_cdd_esch_case IN LOCAL MODE

ENTITY Case

CREATE BY \_Log

FIELDS (esch_case_keyesch_seq_noentry_dateentry_timemessage_typemessage_textcreated_bycreated_atlast_changed_bylast_changed_at )

WITH VALUE #( (

%target = VALUE #( (

esch_case_key = ls_keys-%param-esch_case_key

%control-esch_case_key = if_abap_behv=>mk-on

esch_seq_no = lv_next_seq_no

%control-esch_seq_no = if_abap_behv=>mk-on

entry_date = '20200721'

%control-entry_date = if_abap_behv=>mk-on

entry_time = '082500'

%control-entry_time = if_abap_behv=>mk-on

message_type = 'I'

%control-message_type = if_abap_behv=>mk-on

message_text = 'Payee data updated'

%control-message_text = if_abap_behv=>mk-on

created_by = 'CB0000000170'

%control-created_by = if_abap_behv=>mk-on

created_at = zv_tsl

%control-created_at = if_abap_behv=>mk-on

last_changed_by = 'CB0000000170'

%control-last_changed_by = if_abap_behv=>mk-on

last_changed_at = zv_tsl

%control-last_changed_at = if_abap_behv=>mk-on ) ) ) )

MAPPED mapped

FAILED failed

REPORTED reported.