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This can be a problem with Android devices, when you don't buy them quite New. They are, if ever Registered to a Google address, possibly stuck to that Google address forever after... though I'm not quite sure of this, it is quite often a difficult point.
Was your tablet bought New? or Used? (overtly at least.) If Used, you may have to contact the former owner, perhaps.
I hope you have not already changed your Google password ***** that can lead to problems, if you have. I advise you not to.