You would need to have the original owner request the SIM unlock through T-Mobile.  Their account would need to meet all of the SIM Unlock Policy requirements for hte device and the account.  You would also need their active T-Mobile SIM in the device for the SIM unlock to approve.  

SIM Unlock Policy | Unlock Your Mobile Wireless Device | T-Mobile

You are more than likely better off returning the device to them for a refund.  

if you purchased it online and it was never used on TMO they wont be able to unlock it..TMO have requirements that must be met.

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

if you purchased it from someone else advertised as unlocked then i would return it. safe to say it is not.

It is from the sprint company, it is the same process, I imagine. 

Being a Sprint device and Sprint no longer existing.  I would definitely return it.  It has been a while since the Sprint systems even existed and everyone that remained on Sprint should be ported in to T-Mobile by now.  As an alternative.  Visit your local cellphone repair shop to see if there are any tricks they can do to SIM unlock it for a fee.  

Check to see how much an unlock code costs from a 3rd party vendor. Might be less of a headache.