Cannot be released because it is locked to your account. Please contact AWS Support to unlock it.

Yes, I already removed the rDNS record, but still cannot release the EIP.

shell $: host 18.138.215.68 68.215.138.18.in-addr.arpa domain name pointer ec2-18-138-215-68.ap-southeast-1.compute.amazonaws.com.

My account is free tier, and the note on "support case page" say, "If you have Basic Support, you can't create a technical support case."

So, what can i do now? Thanks.

I just tested the support instructions on the knowledge center article and was able to open a ticket following the instructions there with a free tier account. Can you please try?

Using AWS Support
If you're unable to remove the request using the Amazon EC2 console or the AWS CLI, then request AWS assistance using the steps below:

1.    Open the Request to remove email sending limitations form (https://aws.amazon.com/forms/ec2-email-limit-rdns-request).

2.    Complete the form using the following information:

Email Address: Your email address.

Use Case Description: Your specific use case for requesting rDNS removal.

Elastic IP address: A list of your Elastic IP addresses.

Reverse DNS record: This field is optional.

Reverse DNS Record for EIP 1: Enter please remove rDNS.

Reverse DNS Record for EIP 2: Leave blank.

3.    Choose Submit.
Note: Removing the rDNS might take a few days to propagate through the system