Unknown user permission: ManagePartners error means, you have enabled Manage external user permission in Profiles or Permission sets but not enabled Digital experience in target org.
Answer: 1
If your not using Digital experience in your org, then remove this permission from your profile or permission set.
Profile/ permission set --> app permissions --> Partner relationship Management section --> Manage external users.
Answer: 2
If your using digital experience features, like communities, sites, forums then enable this in settings.
search Digital in setup --> settings --> enable it.
