You bought a used iPhone 11, correct?
Ask the seller what carrier they were using.
Or, tap Settings > General > About and look for information near Physical SIM to see if the Network or Carrier name is displayed there.
If you can find out what carrier the phone is locked to, you will need to contact the carrier to ask them to unlock the phone.
Unfortunately, the carrier will not do this if the previous owner was not up to date on their payments, or if the phone was reported lost or stolen by the owner.
If this is the case, all that you can do is contact the seller and ask for your money back. Unfortunately, in most cases like this, the seller has disappeared.