I locked my SIM card and I don’t know what to do

Well you didn't bother to tell us it's used iPhone. Only the carrier the phone is locked to can unlock it. You can't. We can't. Apple can't. The carrier can.

How can someone sell a phone and not know who they paid for cellular service? That makes no sense. You may have been scammed here.

See this link about unlocking from a carrier lock, but if you have no success, I'd return the phone for refund --> How to unlock your iPhone for use with a different carrier - Apple Support

Also read this about buying used devices --> The All Too Common SAD Reality of Buying … - Apple Community

What if you can’t. Cause the previous owner didn’t tell me sim was locked he got it for his wife before she passed an I bought it an he don’t remember what carrier I have straight talk an it won’t accept my sim card

Contact your cellular provider for help.