Why am I not able to find users to update the name in assignee field for Object workflow

Summary:

We have created an Object workflow with Business process flow to define the approval process of Organization(Accounts) in Customer Data Management.
In the workflow actions when we are trying to update the approver filed after updating the Business process flow, the users we try to update doesn't show up in search.

Is there any specific role that user must have to appear here? can we restrict there access for OOTB entities?

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