Using entity_lookup inside an entity_generate migration process plugins

I'm trying to use an entity_lookup process plugin inside an entity_generate process plugin. The YML looks like this

field_validation:
    plugin: entity_generate
    access_check: false
    entity_type: node
    source: validation_name
    value_key: title
    bundle: validation
    ignore_case: true
    values:
      title: validation_name
      body: validation_description
      field_validation_type:
        plugin: entity_lookup
          access_check: false
          ignore_case: true
          source: validation_type

The error I'm getting is

Uncaught PHP Exception Drupal\Core\Config\UnsupportedDataTypeConfigException: "Invalid data type in config xxxxxx, found in xxxxxx : A colon cannot be used in an unquoted mapping value at line 282 (near " source: validation_type")." at xxxxxx line 117, referer: xxxxxx

Because I'm relatively new in using the Migrate/Migrate Plus modules, I'm not sure if the error is triggered because I'm doing something wrong with the syntax or just because I can't use entity_lookup inside an entity_generate. So, the question is: can I do something like this, or I'm forced to use a separate migration instead of entity_generate and the call it with migration_lookup?

my field definition:

-
 name: validation_type
 label: 'Validation Type'
 selector: $.Description.Validation[*].Type #item_selector is set to $.Schema[*]

the relevant part of the source JSON:

{
  "Schema": [
    {
      "Description": {
        "Validation": [
          {
            "Type": "CUSTOM_TYPE",
            "Description": "lorem ipsum"
          }
        ]
      }
    }
  ]
}