Employers may  fire people because they cannot do the job. Employers do need to make reasonable accomodations for people with disabilities or conditions that would legally qualify as disabilities, but the question is what is "reasonable." If it's impossible for a CNA to safely (his or her safety) work in this particular environment/for this particular employer/at this particular job if he or she were allergic to vinyl powder free gloves, then it might be legally acceptable to have terminated you; however, if there are reasonable ways around it--buy different gloves for example--you might have a cause of action. If you think the latter might be the case, you  should consult with an employment law attorney to see what recourse you might have.