When attempting to edit a constituent relationship, a user may receive Error: Data could not be saved. Cannot insert the value NULL into column 'SOURCECONSTITUENTID', table 'dbo.REVENUERECOGNITIONDEFAULT'; column does not allow nulls. INSERT fails.
This was a data/conversion issue as the relationship had the matching gift check box marked but no entry in the REVENUERECOGNITIONDEFAULT table. The easiest fix would be to edit the relationship, uncheck the box 'The organization will match contributions for this relationship', save the relationship. Edit the relationship and mark the box if necessary.
If above does not work, you may try to delete the relationship and add it back.