Legal age to buy bitters?

Hi! I’m 16f and work in a foodhall that has a bar. I have a medical condition and am basically nauseous all the time so I take zofran as needed. I ran out of my prescription this week and my mom told me to go to the bar and ask for ginger ale with two splashes of bitters. I have been doing that for the past few days while I’m at work, and it’s helped tremendously. Everyone at work knows how old I am. Today a different bartender was there and he refused to sell it to me because I’m not 21. He said the alcohol content was too high. I explained that it’s legal and sort of like if I was ordering ginger ale with vanilla extract in it. He still refused. What can I do?

Edit: for clarification most bitters, like Angostura, have an alcohol content of around 35-45% abv and pure vanilla extract typically contains between 35% and 50% abv