If you're a Tmobile USA customer and meet the requirements in the link below you can request an unlock code.

https://www.t-mobile.com/support/devices/unlock-your-mobile-wireless-device

Thanks, @gramps28. The phone was sent from USA to me in Liberia, West Africa. I saw a T-mobile SIM in the phone .

There's an application in the phone called "Network Unlock" I tried using it be is says "Couldn't Unlock Your Phone"

I have been looking at videos online to help me but it's not possible and that's why I joined this community to get a help.

Your device can not be SIM unlocked by T-Mobile because you are not a T-Mobile USA customer and because you do not meet the SIM Unlock Policy requirements.  You could try a third party unlocking service online or locally at a cellphone repair shop.  They will charge a fee to remove the SIM lock.

Okay, @syaoran. Thank you so much. I'll be grateful if I can get a reliable third-party unlocking service online. 

Since it's an Android you can try requesting a temporary unlock until you find a permanent solution.

I don't know if you can do that from the phone but it's worth a look.

@gramps28, how can I do a temporary unlock?

A temporary SIM unlock should be an option in the SIM Unlock app.  Only two can be granted in a year.  If the option isn't there, then they might have been used by the person the previously owned the device.  Temporary unlocks expire after 30 days.  

We also have a Samsung A15 that we purchased in Rome due to a screen break of an old LG. We immediately put our TMobile sim in it and set it up in the shop there. Sadly we recently learned that Samsung phones have regional locks and  will not work in counties outside of their origin due to regional locks imposed by Samsung, if anyone knows how to get around this please advise. We have spent hours on the phone with T-Mobile and Samsung and been given contradictory information and we’re made promises that haven’t been fulfilled. Considering just changing carriers and starting from scratch as it has been a frustrating ordeal.