You can use this steps to unlock your device:

Log into your T-Mobile Account and choose your line on the Accounts page.

Choose Check device unlock status to check if your device is locked or unlocked.

Under the device image, check if your device is eligible to be unlocked. Review Device Unlock Policy for a list of eligibility requirements.

Make sure you are connected to the T-Mobile network or Wi-Fi.

Select your device type and follow the steps

Thanks a lot for your time and I see here that your device is 100% paid off and is one step away from getting unlock.
 
For reference to unlock a device we need the following eligibility met:
 
1. The device must be paid off 
2. The device must have 40 days usage
3. The device must be purchased from T-Mobile
4. The account must be current
 
We have the first three eligibilities met, only that 4th one is yet to be work on, just to ask- may I know do you have a date in  mind to clear of your account past due?