No, if you quit--voluntarily leave work--you will not get unemployment. What you should do is contact the Equal Employment Opportunity Commission (EEOC) to file a disability discrimination complaint. The law requires employers to make "reasonable accommodations" to employee medical issues--that is, changes in rules or policy which are not expensive or disruptive to allow the employee to work. Letting you sit is neither expensive nor disruptive--your employer appears to be violating the law. File a complaint; you may be entitled to compensation, or at least an order  requiring the employer to let you sit.