Deployment error "The service "private_message.notifier" has a dependency on a non-existent service "message_notify.sender"."

Hi, thanks for reporting this.
Are there any steps to reproduce?

Working on a drupal 8 migration to Drupal 9 and encountered this error:

The website encountered an unexpected error. Please try again later.
Symfony\Component\DependencyInjection\Exception\ServiceNotFoundException: The service "private_message.notifier" has a dependency on a non-existent service "message_notify.sender". in Symfony\Component\DependencyInjection\Compiler\CheckExceptionOnInvalidReferenceBehaviorPass->processValue() (line 31 of D:\wamp64\www\lcms8to9\vendor\symfony\dependency-injection\Compiler\CheckExceptionOnInvalidReferenceBehaviorPass.php).
Symfony\Component\DependencyInjection\Compiler\CheckExceptionOnInvalidReferenceBehaviorPass->processValue(Object, ) (Line: 60)
Symfony\Component\DependencyInjection\Compiler\AbstractRecursivePass->processValue(Array, ) (Line: 28)
Symfony\Component\DependencyInjection\Compiler\CheckExceptionOnInvalidReferenceBehaviorPass->processValue(Array) (Line: 67)
Symfony\Component\DependencyInjection\Compiler\AbstractRecursivePass->processValue(Object, 1) (Line: 28)
Symfony\Component\DependencyInjection\Compiler\CheckExceptionOnInvalidReferenceBehaviorPass->processValue(Object, 1) (Line: 60)
Symfony\Component\DependencyInjection\Compiler\AbstractRecursivePass->processValue(Array, 1) (Line: 28)
Symfony\Component\DependencyInjection\Compiler\CheckExceptionOnInvalidReferenceBehaviorPass->processValue(Array, 1) (Line: 39)
Symfony\Component\DependencyInjection\Compiler\AbstractRecursivePass->process(Object) (Line: 140)
Symfony\Component\DependencyInjection\Compiler\Compiler->compile(Object) (Line: 789)
Symfony\Component\DependencyInjection\ContainerBuilder->compile() (Line: 1335)
Drupal\Core\DrupalKernel->compileContainer() (Line: 916)
Drupal\Core\DrupalKernel->initializeContainer() (Line: 477)
Drupal\Core\DrupalKernel->boot() (Line: 707)
Drupal\Core\DrupalKernel->handle(Object) (Line: 19)

composer.json content:

{
"name": "drupal-composer/drupal-project",
"description": "Project template for Drupal 8 projects with composer",
"type": "project",
"license": "GPL-2.0-or-later",
"authors": [
{
"name": "",
"role": ""
}
],
"repositories": [
{
"type": "composer",
"url": "https://packages.drupal.org/8"
},
{
"type": "vcs",
"url": "https://github.com/dakala/php-sdk.git"
}
],
"require": {
"php": ">=7.0.8",
"composer/installers": "^1.2",
"constantcontact/constantcontact": "dev-development",
"cweagans/composer-patches": "^1.6.5",
"drupal/addtoany": "^1.12",
"drupal/admin_toolbar": "2.5",
"drupal/backup_migrate": "^4.1",
"drupal/better_exposed_filters": "5.0",
"drupal/block_class": "^1.1",
"drupal/bootstrap": "^3.21",
"drupal/captcha": "^1.0",
"drupal/composer_deploy": "1.6",
"drupal/console": "^1.0.2",
"drupal/constant_contact_block": "^1.0",
"drupal/copyprevention": "^1.1",
"drupal/core": "^8.8.0",
"drupal/core-composer-scaffold": "^8.8.0",
"drupal/ctools": "^3.2",
"drupal/date_popup": "^1.0",
"drupal/date_popup_timepicker": "^1.0@alpha",
"drupal/devel": "^2.1",
"drupal/ds": "^3.5",
"drupal/entity": "^1.0",
"drupal/entity_print": "^2.1",
"drupal/feeds": "^3.0@alpha",
"drupal/field_permissions": "^1.0@RC",
"drupal/flag": "^4.0@beta",
"drupal/fullcalendar_view": "^2.6",
"drupal/group": "^1.0@RC",
"drupal/jquery_ui_slider": "1.1",
"drupal/markup": "1.x-dev",
"drupal/menu_link_attributes": "^1.1",
"drupal/message": "^1.2",
"drupal/message_notify": "^1.2",
"drupal/migrate_file": "^1.1",
"drupal/migrate_tools": "^4.5",
"drupal/migrate_upgrade": "^3.1",
"drupal/module_filter": "^3.1",
"drupal/og": "^1.0@alpha",
"drupal/owlcarousel": "^1.0@alpha",
"drupal/page_manager": "^4.0@beta",
"drupal/panels": "^4.4",
"drupal/pathauto": "^1.6",
"drupal/private_message": "^2.0@beta",
"drupal/recaptcha": "^3.0",
"drupal/rules": "^3.0@alpha",
"drupal/search_api": "^1.15",
"drupal/search_api_db": "^1.15",
"drupal/sharethis": "^2.0@beta",
"drupal/social_share": "^2.0@beta",
"drupal/superfish": "^1.3",
"drupal/token": "^1.6",
"drupal/user_registrationpassword": "^1.0@alpha",
"drupal/video_embed_field": "^2.2",
"drupal/views_bulk_operations": "^3.5",
"drupal/views_contextual_filters_or": "^1.2",
"drupal/views_slideshow": "^4.6",
"drupal/votingapi": "^3.0@beta",
"drupal/webform": "^5.19",
"drupal/webform_migrate": "^1.1",
"drush/drush": "^9.7.1 | ^10.0.0",
"vlucas/phpdotenv": "^4.0",
"webflo/drupal-finder": "^1.0.0",
"zaporylie/composer-drupal-optimizations": "^1.0"
},
"require-dev": {
"drupal/core-dev": "^8.8.0"
},
"conflict": {
"drupal/drupal": "*"
},
"minimum-stability": "dev",
"prefer-stable": true,
"config": {
"sort-packages": true,
"allow-plugins": {
"composer/installers": true,
"cweagans/composer-patches": true,
"drupal/console-extend-plugin": true,
"drupal/core-composer-scaffold": true,
"zaporylie/composer-drupal-optimizations": true
}
},
"autoload": {
"classmap": [
"scripts/composer/ScriptHandler.php"
],
"files": ["load.environment.php"]
},
"scripts": {
"pre-install-cmd": [
"DrupalProject\\composer\\ScriptHandler::checkComposerVersion"
],
"pre-update-cmd": [
"DrupalProject\\composer\\ScriptHandler::checkComposerVersion"
],
"post-install-cmd": [
"DrupalProject\\composer\\ScriptHandler::createRequiredFiles"
],
"post-update-cmd": [
"DrupalProject\\composer\\ScriptHandler::createRequiredFiles"
]
},
"extra": {
"composer-exit-on-patch-failure": true,
"patchLevel": {
"drupal/core": "-p2"
},
"drupal-scaffold": {
"locations": {
"web-root": "web/"
}
},
"installer-paths": {
"web/core": ["type:drupal-core"],
"web/libraries/{$name}": ["type:drupal-library"],
"web/modules/contrib/{$name}": ["type:drupal-module"],
"web/profiles/contrib/{$name}": ["type:drupal-profile"],
"web/themes/contrib/{$name}": ["type:drupal-theme"],
"drush/Commands/contrib/{$name}": ["type:drupal-drush"]
},
"enable-patching": "true",
"patches": {
}
}
}

It seems I had private_message enabled but message_notify was NOT enabled when I update private_message. I was then unable to get to the GU to enable message_notify.

very short term solution is to just remove/comment out line 43 of private_message.services.yml, enable message, message_notify, clear cache and remove comment.

This is a duplicate of #3184550: Updating from 1.x to 2.x breaks site.
That issue has better description and was created much earlier, so I'll keep it and mark this as the duplicate.