I had a tpo in Nevada in 2015 it is now 2021 was never convicted in court can I buy a firearm

Your facts are a bit vague -- you should call any competent criminal defense counsel, who should know the relevant rules off the top of their head, will advise if there is any problem and, if so, can give basic guidance for how to seal records if required.  Also consider looking at the forms at the Civil Self-Help Center run by the Legal Aid Center of Southern Nevada.