'Invalid Payment Method' error while posting HIT - AWS and payment method already linked

To resolve the "Missing Valid Payment Method" error when posting HITs on Amazon Mechanical Turk, there are several steps you need to take:

1. Ensure your payment method is properly verified in your AWS account. Sometimes even though a payment method is linked, it may not be fully verified. Check your AWS Billing console under Payment preferences to confirm the verification status.

2. For first-time MTurk requesters, you need to request a quota increase for your Mechanical Turk spending limit. Without this increase, your first attempt to publish a HIT will be denied with an error message about exceeding your monthly credit limit.

3. To request a quota increase:

• Sign in to the AWS account associated with your MTurk Requester account
• Navigate to the Service Quotas console
• Search for the Mechanical Turk quota
• Enter your total projected monthly spend amount for Mechanical Turk
• Provide justification if required

4. It's recommended to request the maximum projected amount you'll need rather than submitting separate requests for individual projects.

5. Make sure you've completed identity verification by phone for your AWS account.

If you've already taken these steps and are still experiencing issues, you may need to contact AWS Support by creating a case under the "Account and Billing" category to address your specific situation.
Sources
Request Mechanical Turk quota increase | AWS re:Post
Survey billing help | AWS re:Post