This issue can occur with constituents that have multiple email addresses, with none of them marked as primary. Try marking the email address as primary in RE, then refreshing the constituent list in BBNC.

To do this in bulk, should multiple constituents have this issue, the best way to add a primary email flag is to export the records you need to update using the criteria in the export below, then re-import it after setting the Is Primary value to Yes in the import file.

Screenshot of a constituent export in RE