Unlock phone

Contact your carrier to get it unlocked. See here: https://support.apple.com/en-us/HT201328

Most likely your phone is locked to a carrier, and when you got it someone had previously hacked it to bypass the carrier lock. When you restored it that removed the hack, which probably cannot be hacked again. You will have to find out which carrier it is locked to and contact them to learn their unlocking policy.

Only your carrier can remove their lock.

Call them.

You can try the Contact Support link at the top of this page and ask. Sometimes they will tell you. It’s hit or miss. You can also check the model number of the phone, which will tell you which country the phone was intended for sale it, then ask each of the major carriers in that country. If you post the model number one of us can look it up, or you can google it.

What is the exact message you are seeing?

What has changed?

Phone recently updated? Restored?

If you can't use a different sim then your device is carrier locked.

Did you recently update or restore the phone?

And now you are seeing the message as below? If so, your phone was previously unlocked by hacking. It has now been restored to the non-hacked state, and locked to the original cellular provider.