Nature of transaction is mandatory for most declaration categories and must be specified at either header or item level. If this information differs across items, the field has to be filled for all items, otherwise it should be declared at header level.
Further information from HMRC can be found here: Nature of Transaction
To enter the value at header level, go to the Financial overview tab and locate the Invoice data section.
To enter the value at item level, open each item, and then on the Item tab locate the Customs details section.