CDS12156Obligation Error: Mandatory Data Element has not been provided or the ID for previous document CLE must match the AcceptanceDate in the format yyyymmdd
HMRC explanation
What to check across the Relation, authorisation and sequence errors
Two elements, or a value and a rule, disagree. Read both elements together: completing one element with a particular code often requires another value, and the fault is frequently in the element you did not change. The family also covers parties and authorisations that are not recognised, references that must be unique, and messages sent out of sequence.
A declaration that clears CDS can still be wrong
The free eligibility checker confirms your import activity and whether a free post-clearance audit applies. No data upload, no commitment.
Source: HMRC, Customs Declaration Service error codes, Open Government Licence v3.0. GOV.UK publication · Licence