Previous owner is unknown, how do I unlock Ipad?

Without the cooperation of the previously authorized owner, you can't.

You just don't. And you now are the proud owner of a useless device. See this user tip I wrote --> The All Too Common SAD Reality of Buying … - Apple Community

You don't.

If you can, return this iPad and get your money back.

You have a dead, useless, electronic paperweight with NO WAY EVER to get that iPad working.

Good Luck to You!