When a user tries to add an existing Shared Field to a Document Subtype or Classification, the Shared Field is not appearing under the options for Existing Shared Fields.
The Shared Field is already being used on the Document Type above that Document Subtype. If it cannot be added to a Classification, it is already being used on either the Document Type or the Subtype above the Classification.
The user can remove the field from the Document Type or Subtype and apply it directly to the Subtype or Classification if needed.
Vault Document Fields Documentation: Managing Document Fields