Have a doctors note stating I can’t wear face coverings, work is making me. Says I have to show up for work with coveringor get fired. Is that legal?

This is a really untested area of law. The face coverings are for the protection of others as much as yourself. The employer would have a strong argument that the health and safety of others in preventing the spread of COVID is of greater importance than your shortness of breath. I can see disagreement on this, as it comes down to a policy consideration.