Failed to load resource: the server responded with a status of 429 (Too Many Requests) Error

I try to get a day in an appointment site, but I cannot set an appointment day, and in the console part of the web page appears the follow message "Failed to load resource: the server responded with a status of 429 (Too Many Requests)", What Does it mean? and, How Can I fix it to get an appointment day through client side?, because I think is a problem related of my side. Thanks in advance