No database connection configured for source plugin uc6_product_type

Thanks for the response. I don't have a fallback database connection setup. I'll have to try that. What I don't understand is why it would be trying to use the fallback connection when everything else is using the connection I setup when starting the migration using Drush just fine. One thing I found interesting looking at the YAML configurations for the migrations that failed is that each have 'upgrade_uc6_product_type' listed as both required and optional. Not sure if that's a problem. Here's the configuration for upgrade_d6_field_instance.

uuid: ef6eeccc-06bb-4fe2-9a94-b1621a4d441a
langcode: en
status: true
dependencies: {  }
id: upgrade_d6_field_instance
class: Drupal\migrate_drupal\Plugin\migrate\FieldMigration
field_plugin_method: alterFieldInstanceMigration
cck_plugin_method: null
migration_tags:
  - 'Drupal 6'
  - Configuration
migration_group: migrate_drupal_6
label: 'Field instance configuration'
source:
  plugin: d6_field_instance
  constants:
    entity_type: node
process:
  field_type_exists:
    -
      plugin: migration_lookup
      migration: upgrade_d6_field
      source:
        - field_name
    -
      plugin: extract
      index:
        - 1
    -
      plugin: skip_on_empty
      method: row
  entity_type:
    -
      plugin: get
      source: entity_type
  field_name:
    -
      plugin: get
      source: field_name
  label:
    -
      plugin: get
      source: label
  description:
    -
      plugin: get
      source: description
  required:
    -
      plugin: get
      source: required
  status:
    -
      plugin: get
      source: active
  settings:
    -
      plugin: d6_field_field_settings
      source:
        - widget_type
        - widget_settings
        - global_settings
  default_value_callback:
    -
      plugin: get
      source: ''
  default_value:
    -
      plugin: d6_field_instance_defaults
      source:
        - widget_type
        - widget_settings
  node_type:
    -
      plugin: migration_lookup
      migration: upgrade_d6_node_type
      source: type_name
  product_node_type:
    -
      plugin: migration_lookup
      migration: upgrade_uc6_product_type
      source: type_name
  bundle:
    -
      plugin: callback
      source:
        - '@node_type'
        - '@product_node_type'
      callable: array_filter
    -
      plugin: callback
      callable: current
    -
      plugin: skip_on_empty
      method: row
destination:
  plugin: 'entity:field_config'
migration_dependencies:
  required:
    - upgrade_d6_node_type
    - upgrade_d6_field
    - upgrade_uc6_product_type
  optional:
    - upgrade_d6_field
    - upgrade_d6_node_type
    - upgrade_uc6_product_type

I added the fallback connection and the migrations worked. I'm still unsure why the fallback was needed for these migrations.

I wonder if this has something to do with the fact that it's trying to run the migration called 'uc6_product_type' instead of 'upgrade_uc6_product_type'.

As mentioned in the original post, the following migrations produce the error Migration did not meet the requirements. No database connection configured for source plugin uc6_product_type

upgrade_d6_field_instance
upgrade_d6_field_instance_widget_settings
upgrade_d6_view_modes
upgrade_d6_field_formatter_settings

Thanks again for your help with this!

I believe this issue might be causing another issue: https://www.drupal.org/project/commerce_migrate/issues/3036242

Interestingly, I get the following error in drush if I try to import upgrade_d6_field_instance as the very first migration.

Migration upgrade_d6_field_instance did not meet the requirements. Missing migrations uc6_product_type, upgrade_d6_field, upgrade_d6_node_type, upgrade_uc6_product_type.

I find it interesting that both uc6_product_type and upgrade_uc6_product_type are included in the requirements.

If I start running the missing migrations in order, the uc6_product_type will process 9 records while the upgrade_uc6_product_type migration will ignore the same 9 records. I see tables called migrate_map_uc6_product_type and migrate_map_upgrade_uc6_product_type.

When I then try to run upgrade_d6_field_instance after migrating all its requirements, it ignores all 33 records. My hunch is that it has something do with there being both a uc6_product_type and upgrade_uc6_product_type required.