You have to tell them it is a violation of your HIPAA rights for them to ask you to wear a mask as well as to as what medical condition you have that restricts your ability to wear a mask. The “private property” comment is insane because every place is private property unless it’s government... I tell them that and they usually tell me I can speak to their manager and I say “thanks I will on my way out”. That has worked for me so far, some have a major power trip though and will dig their heels in... you have to hold them accountable though, meaning force them to get their boss and say the same thing calmly and nothing else. Don’t let them get you angry or upset, stay calm and they generally give in because it truly is a power trip for them... nothing else