Complex sentence: The U.S. Congress could have blocked this by passing legislation within 45 days, but no member of Congress even introduced such legislation, and Cuba was officially removed from the list on May 29,2015.
Split into simpler sentences: The U.S. Congress could have prevented the removal of Cuba's designation by passing legislation within 45 days. Congress did not act, and Cuba was officially removed from the list on May 29,2015.