In most of cases account gets automatically activated once you do payment.
As this is community forum, DO is not able to provide account assistance here.
You will have to contact support directly via site or from Control Panel.
They response fast and will resolve or try to help you resolve it.

Hey @lethitragiang910,

In our public community, we aim to answer open questions about anything SysAdmin, DigitalOcean and beyond. However, we make every attempt to keep personal information safe and so don’t ever access personal account information here. This means we can’t provide help with any account or billing-related issues.

Please reach out to our amazing support team who will be more than happy to assist you with your account issue! :)

https://www.digitalocean.com/support/

Hope that helps!
- Alex.