I found the cause for this. It can’t create the site you are asking for and doesn’t tell you why ??
Click “Sites” in your sidebar and hover over the name of the subsite you hare created. At the bottom of the browser you will see the url. The id in the url is what this plugin wants. It should show as a numerical value, not the unique name of the site.
For more information reference https://wpengine.com/support/multisite-how-to-add-new-sites/.
