Header and item relation errorsRelation & missing element
CDS11006Relation Error: Net Mass must be lower than Gross Mass
HMRC explanation
What to check across the Header and item relation errors
Values across the declaration do not agree, for example the total number of items against the items declared, or net mass against gross mass. Check the header totals against the item lines, the trade movement and declaration type against the requested procedure, and whether the element you are amending can be amended on this declaration type.
A declaration that clears CDS can still be wrong
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Source: HMRC, Customs Declaration Service error codes, Open Government Licence v3.0. GOV.UK publication · Licence