What’s the floor made of? Cement? Wood? Is it just vibrations coming through the floor that the neighbor is complaining about? Or is it just generally that you are practicing and the sound is coming through everywhere(open windows, through the floor, etc.)?
Try as much as possible to work with the neighbor. When are they not there? Ask them when are the best times of the day or evening for you to practice? Tell them how much time you need to practice each day and then ask when would be the best times to do that? You may have to put sound absorbing material and carpeting on the floor, but also on the walls near the harp to minimize the sound.