Unable to merge the candidate is invalid, merge only valid candidate types

Summary:

When tried to merge the external candidate with the existing contingent worker getting error as ""The candidate merge failed because of this issue: The candidate merge request is invalid. You can only merge valid candidate types. (IRC-1590545)"."

Content (please ensure you mask any confidential information):

We have an existing contingent worker in system.
New external candidate applied for new job requisition where this candidate data matches with the existing contingent worker.
So we tried merging this external candidate with this existing contingent worker in system.
But we received error saying "The candidate merge failed because of this issue: The candidate merge request is invalid. You can only merge valid candidate types. (IRC-1590545)".