You must meet all the criteria listed in this link

https://www.t-mobile.com/responsibility/consumer-info/policies/sim-unlock-policy

specifically

Unlock eligibility requirements

We unlock mobile devices that meet the following eligibility criteria free of charge.

Device eligibility is determined as follows:

• The device must be sold by T-Mobile.
• The device must not be reported as lost, stolen or blocked.
• The account associated with the device must be in good standing.
• The device must also satisfy all the Postpaid or Prepaid unlocking requirements outlined below.

Unlock eligibility for mobile devices on Postpaid plans

• The device must have been active on the T‑Mobile network for at least 40 days on the requesting line.
• If the device was financed or leased through T‑Mobile then all payments must be satisfied and the device must be paid in full.
• If the device is associated with a canceled account, then the account balance must be zero.
• T‑Mobile may request proof of purchase or additional information in its discretion and certain other exceptions may apply.

Unlock eligibility for mobile devices on Prepaid plans

• At least  365 days  must have passed since the device was activated on the T Mobile network.
• If the device was activated less than 365 days ago, then the   Prepaid account associated with the device must have had more than $100 in refills during that period for each line active on the account.  In   addition, more than 14 days must have passed since the device was purchased.
• No more than 2 mobile device unlocks have been complete per line of service in the last 12 months.
• T‑Mobile may request proof of purchase or additional information in its discretion and certain other exceptions may apply.

Out of curiosity what type of phone is it?