How can I request a device unlock?

The original owner must handle this - you can't.  Have them process the account removal and unlock. 

is there any way I can talk to an at&t agent online?

No only the original owner can do it. Get a refund or have the seller unlock it 

Hey there @Hercullesviana, we see that you're interested in unlocking your device that you'd purchased from a third party.

Allow us to share the necessary steps you'll need to take in order to get your device successfully unlocked.

As @lizdance40 shared, In order for us to provide you with further assistance, we'd need to get in contact with the original owner as the device has to be unlocked from their account otherwise this goes against AT&T's policy and guidelines. We can't override the system to do so.

You do have the option to wait for the device unlock portal to reset itself and resubmit your device unlock request. We understand this isn't a preferred method but this will be the only alternative if you're unable to get further assistance from the previous owner.

We sincerely apologize for the inconvenience this has caused you.

If you have additional questions, please do share more with us.

Thank you for connecting with the Community!

Jonye, AT&T Community Specialist