Hi, thank you for contacting JustAnswer.com. My name is Russell. I will do my best to provide the right answer to your question.
In a case like this, the first thing to ask is, Were you sold this tablet as Used, or as a New, tablet?
That explains that then: your newly-purchased tablet was obviously used by a person with an unknown Google account.
I honestly do not know how to 'unlock' this requirement that the reset be completed by entering former credentials for a Google account. I have to Opt Out, opening the case for other Experts in this category.