CDS77005Reason for waiver must be declared for Document Category

HMRC explanation

Reason not given for use of a 'Y' code
The Y series document code used requires a document reason. Check the 'Conditions' box in the Online Tariff or appendix 5A for the information required.

Source: HMRC, Customs Declaration Service error codes, Open Government Licence v3.0. GOV.UK publication · Licence

What to check across the Document obligation errors

A document code needs further detail that is missing: an identifier, a status code, a quantity, a consignment reference, or a reason for a waiver. Check every document code on the item carries the details HMRC's explanation lists for it.

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