To resolve this issue, use the script below to transfer the Luminate Online Survey Results from one constituent to the other then attempt the merge again. Just change the Constituent lookupids used for the source and target constituents.

DECLARE @SOURCECONSTITUENTID uniqueidentifier;
DECLARE @TARGETCONSTITUENTID uniqueidentifier;

select @SOURCECONSTITUENTID = ID
from dbo.CONSTITUENT
where lookupid = '8-12345678'; --insert source constituent lookupid

select @TARGETCONSTITUENTID = ID
from dbo.CONSTITUENT
where lookupid = '8-87654321'; --insert target constituent lookupid

UPDATE USR_LUMINATE_APPSURVEYRESPONSE
SET CONSTITUENTID = @TARGETCONSTITUENTID
WHERE CONSTITUENTID = @SOURCECONSTITUENTID;