EC2 reserved instance stuck in Payment Pending

First check from this URL, if you see the reserved instance, which you were trying to purchase, if you see here, well and good.

Note: Change region in the URL as accordingly.

If you don't see the purchased RI there, then:

Here are the steps to open a support ticket with AWS:

1. Navigate to the AWS Management Console.
2. In the navigation bar on the upper-right, choose Support, and then select Support Center.
3. Select Create case.
4. Choose Account and billing.
5. Provide the details which you shared here and any additional context, which you can give.
6. Click Submit.

Support engineer would be able to help you out and confirm if there is anything required from your side or not.