The error “externalCustomerid does not equal existing externalCustomerId” occurs when you are trying to merge two Customer Profiles that are linked to two different master data/lookup adapter.
Double-check that you are merging the Profiles for the same person as this merges the Profiles in Gladly, not your master data.
To fix the issue, you must unlink one of the Profiles using the Unlink Customer option. Once unlinked, you can merge the Profiles again.