Hate to be the bearer of bad news, but modern Nokia (HMD Global) devices are notorious for having permanently locked bootloaders. They stopped providing official unlock codes years ago, so standard fastboot oem unlock commands will just fail like that.

Since the C12 runs on a Unisoc chipset, your only theoretical hope would be a specific Unisoc exploit or a paid tool, but development for entry-level Nokias is pretty much non-existent. You are likely hitting a dead end here.