Why does new iphone se say no SIM card

See if the following article can help, otherwise take it back to T-Mobile. It may be possible that SIM card they provided you is defective.

• If you see 'invalid SIM' or 'no SIM' on your iPhone or iPad - Apple Support

Could be a number of reasons:

1. It is not activated.
2. It does not have a SIM card installed.
3. It has the wrong SIM card.

I am assuming that you got a new 2nd gen iPhone SE ... correct? Did you purchase it directly from Apple or another third-party source?