Error:
'Merge person has failed validation because Person xxxxxx is related to person xxxxxxx'
To fix this, you'll need to delete the relationship manually in the front end. The Person Merge guidance advises cleaning up relationships either before or after the merge.
If both the source and target person are related to the same individual, make sure to tidy up those relationships manually as part of the process.
If the above resolution does not resolve this issue, please raise a new case and reference the title of this article.