I bought an iPhone from someone who didn’t unlock it first

Unfortunately, that’s not possible.

Apple values your privacy and has safeguards in place to ensure that no one can access the device.

You will need to reach out to the owner because it is the owner’s responsibility to properly erase the device before selling it.

Until then, it is locked. 🔐

you will have to reach out to the previous owner and hope they are willing to reset it for you. Good luck