This error can be caused by no initial email status being set on the record. If you attempt to modify particular email interests, but the user has not been set to accept or not accept email then this error may occur. If you would like to unsubscribe this user from all interests and emails in general then:
  1. Constituent360 > Constituents > Search for the constituent record.
  2. Edit Constituent Record > Uncheck the Accept Email box.
  3. Save.
This will opt them out of all emails.  

If the user should only be unsubscribed from certain interests then you can perform the same steps, but instead make sure the box is checked and save. Now that the user is either opted out or opted into emails you can individually save interest changes without an error.