Steps to Reproduce:
In Salesforce Classic
In Lightning Experience
If there are too many User records to do this manually, you may be able to use Data Loader to bulk remove the field values to allow you to deactivate the user. For detailed steps to complete this action, see Unable to deactivate User due to custom User Hierarchy field
Note: Removing the value from these field is at the discretion of the administrator of the particular org. Additionally,
before deactivating a user, make sure to confirm that they are not a default Lead or Case owner and creator in the following areas in Setup:
Note: Users can not be deleted from the organization, only deactivated. For more information, see Delete Users.
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