It is likely the true error is not related to the Constituent Code, but rather some other required field that is missing. Raiser's Edge raises the "required field missing" errors, and we hand them to you.
Unfortunately, when Constituent Code is set to required in RE (it is not required by default), RE will sometimes return the error in that field even when that's not actually the missing field. To get to the "real" error, temporarily turn off the requirement in RE for Constituent Code to be required and then import the rows that were exceptions with the Constituent Code error. RE should then raise the "real" error and you will be able to correct the issue on the incoming rows. You can then re-enable the requirement for Constituent Code in RE.
Please let me know if I can be of further assistance!