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Welcome hassan coliqs,
I understand how frustrating it can be when you would like to unlock a Nokia Lumia 521 that was previously locked to the T-Mobile network.
Before proceeding, please be aware that support for Windows Mobile officially ended in 2019 so at the moment and since this is a user-to-user forum, our role as moderator is to provide troubleshooting steps and workaround for Microsoft products. Since the issue that you're facing is most likely related to mobile service, it is out of our support scope, and we also do not have any access nor authorization to any tools that can help you with unlocking your mobile device. Since you mentioned T-Mobile, I would advise contact T-Mobile directly for further support about their unlocking phone device policy if the device was previously registered to their network.
Thank you for your understanding.