@Tiger Guo Thank you for reaching out regarding the error you are receiving when trying to purchase a custom domain. We apologize for the frustration that you have encountered.
Since you followed Marilee's instructions and it did not resolve the issue the final step left is to create a support ticket as we lack further visibility into why your subscription is hitting this limit here on the forums.
To create a free support ticket, please go here and follow the instructions.
Thank you for your understanding.