Mapping is incomplete

Hi Experts,

Im working on File to IDOC scenario. i have completed the mapping between the source and target structure.

When im trying to activate the message mapping, it is giving the error as mapping is incomplete. but i have mapped all the fields.

Target field mapping '/ZCOOP/IDOC/@BEGIN' incomplete; reason: Mapping for target field '/ZCOOP/IDOC/@BEGIN' incomplete

For segment field at the target side with source field. bu t i Dont have any fields at the source side, so i have created constant field to pass empty value.

Please guide me to resolve the issue.