What if you can’t get in contact with previous owner

Try this to activate your phone: https://support.apple.com/en-us/HT201407.

.

That's the Apple Activation server, and which sometimes gets overrun.

Try again in an hour or two. Maybe try again from a different Wi-Fi.

(Try the Apple activation-troubleshooting article listed above, too.)

Then contact the folks that work for Apple, using the link provided in the message.