What to fill in last name while creating new AWS certification account?

First Name and Last Name are mandatory for AWS Certification. But I do understand the fact that "It is okay to not have a surname"

I believe you are stuck in this page,

Please verify your name before you proceed to your AWS Certification Account. https://www.certmetrics.com/amazon/candidate/verify_name.aspx?msg=true

To resolve this issue, I would suggest you to contact AWS Training and Certification, https://support.aws.amazon.com/#/contacts/aws-training

You may visit the above link and scroll down the page and fill up the form and someone from AWS will respond to you shortly with a solution.