probably need a different phone. there are two types of unlocked, carrier and OEM. flashing a new OS requires both, but most of the used "unlocked" phones for sale are only carrier unlocked and cannot be used for GOS. if the listing doesn't specifically list OEM/factory unlocked then it isn't truly unlocked.
i think i went through like 4 phones my first time trying to set up GOS, which is why the second time i bought direct from g**gle.