Unable to CreateSubscription for Notifications ( SQS, SP-API )

The 403 Forbidden error you're encountering when trying to create a subscription for notifications suggests there's an issue with permissions. Based on your description, there are a few potential causes and solutions:

1. SQS Queue Policy Configuration: Make sure your SQS queue policy properly allows the EventBridge service to send messages to your queue. The policy should include:

• "Service": "events.amazonaws.com" as the Principal
• "Action": "sqs:SendMessage" permission
• The correct Resource ARN for your SQS queue

2. IAM Permissions: The IAM role or user you're using to create the subscription might need additional permissions. Ensure it has the necessary permissions to:

• Create notification subscriptions
• Work with EventBridge rules and targets
• Access the SQS queue

3. Protocol Configuration: When creating a notification subscription, ensure you're using the correct protocol. For SQS, you should be using either "HTTPS" or "SQS" as the protocol value, depending on the specific API requirements.

4. EventBridge Rule Validation: If you've already created an EventBridge rule, verify that it's correctly configured to target your SQS queue.

For the issue of not receiving messages in your SQS queue after changing quantities in Seller Central, this could indicate that either:

• The subscription wasn't successfully created
• The EventBridge rule isn't properly configured
• There might be a delay in processing the notifications

I recommend checking CloudTrail logs for any denied actions related to your SQS queue or EventBridge rules, which can provide more specific information about permission issues.
Sources
CreateNotificationSubscription - Amazon WorkDocs
Resolve EventBridge notification failures in Amazon SQS | AWS re:Post
AMS Subscription Error (eu-west-1): Unable to validate destination SQS queue despite correct policy | AWS re:Post