how to migrate multiple taxonomy terms with entity lookup plugin
The field that you're migrating has multiple values and you're trying to run entity_lookup as though it has only one. I think you just need to make sure you're iterating over the values with the sub_process plugin.
Using this issue post as a reference, along with the docs for entity_lookup:
field_farbzuordnung:
    plugin: sub_process
    source: field_farbe
    process:
        target_id:
        plugin: entity_generate
        source: value
        value_key: name
        bundle_key: vid
        bundle: alle_farben
        entity_type: taxonomy_term
        ignore_case: true

...I think you'd want to do something like
field_states_operating_in:
    -
      plugin: sub_process
      source: field_states_operating_in
      process:
        target_id:
          plugin: entity_lookup
          source: target_id
          entity_type: taxonomy_term
          bundle: states_operating
          bundle_key: vid
          value_key: target_id


I'm not entirely sure that the source and value_key are correct but I think this should get you most of the way there.