SharePoint Designer is a tool which is used to access SharePoint sites. By the name itself one can Infer that it is mainly used for designing purposes. One can write CSS code in the Style library and modify the Master Page. It is a great tool to have unfortunately Microsoft would not be developing any newer versions for SharePoint Designer. The last one is SharePoint Designer 2016.
The first step to developing anything in the tool is to open your SharePoint site. And one must be very careful doing so. While launching the SharePoint Designer tool, it would prompt you to specify a URL. Now this is where one has to be very clear. You cannot specify a URL from any location of your SharePoint site. It has to be the root site.
So suppose your domain name is “coffee” and you happen to put this URL https://coffee.sharepoint.com/SitePages/Home.aspx , it is not going to work. SharePoint Designer will only recognize the root site and not any folders. In this case SitePages is a folder in SharePoint and Home.aspx is a file inside the SitePages folder. One can only specify the root site. So one has to type https://coffee.sharepoint.com/, and this would work 🙂