how to restore a deleted text message on iphone 12?

You don't, unless you have been backing up the iPhone regularly and another backup has not occurred since you deleted the message.

In other words, if the message was backed up......and.....another backup has not run since you deleted the message, the message might still be in the backup.

You would have to erase your phone though, and restore all the data from the backup, so you would lose any data that you have added to the phone since the backup occurred.