I think you have the burden of proof backward here (that is, I think the burden lies with those making the affirmative claim -- namely, those arguing in favor of masking), but I'll bite anyway.
Here are some of the harms:
1. Discomfort in terms of temperature. It is hot outside. I sweat heavily, and the mask only makes the situation worse. In fact, the mask often makes it just gross.
2. Discomfort in terms of breathing. Yes, this is a real issue, especially when the paper (surgical) masks get wet from sweat.
3. Occasional allergies or other bad reactions to masks. From time to time, I have developed coughs or other symptoms from talking, walking, exercising extensively while wearing masks.
4. Communication. I find I communicate better -- and am often actually treated better -- when people can see my face.
5. No need. I was always at low risk, but I'm at lower risk now that I have had five vaccine shots (two Chinese plus three American). Even if masks did help (and again, I don't think the paper ones I wear to meet mandate requirements do much at all), why would I need them? I'm vaccinated.
6. Promoting false solutions and yielding to a climate of fear and ignorance. Just to appease the moderators, let me acknowledge that N95 and similar masks absolutely can work, but the recent real-world data on the effectiveness of mask mandates (which usually prompt people to wear paper or cloth masks) are fairly unimpressive. I will post some links below, but the example of Hong Kong is illustrative -- terrible outbreak despite near 100% mask compliance.
Some links related to point 6, so that the moderators do not accuse me of spreading "misinformation":
https://www.sfgate.com/coronavirus/article/bay-area-mask-mandate-results-17271294.php
(sfgate.com is the San Francisco Chronicle, a major newspaper)
https://www.nytimes.com/2022/05/31/briefing/masks-mandates-us-covid.html