Cannot update assignment field by HDL

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Hello experts,

I should update some custom field related to the Assignment page for some employees by HDL.

I'm using the following files:

METADATA|WorkTerms|AssignmentId|EffectiveLatestChange|EffectiveSequence|EffectiveStartDate|WorkTermsAssignmentId

METADATA|Assignment|AssignmentId|EffectiveLatestChange|EffectiveSequence|FLEX:PER_ASG_DF|Customfield1(PER_ASG_DF=Global Data Elements)|Customfield2(PER_ASG_DF=Global Data Elements)|EffectiveStartDate|WorkTermsAssignmentId

However, I'm continuously incurring in the following error regarding WorkTerms sheet: A parent for this record was not found. Use any of the four supported key-resolution systems to supply a valid parent reference.

Which field should I add in the Work Terms file?

Thanks in advance for your time!