If you don't specify the protocol when adding a site, you are given an inappropriate error message. A 'Site URL' has been specified, but you don't like it
Ideally though we should do what the user wants and:

• try https:// ; and failing that
• try http://

There may be a delay for timeouts etc, but I think that's preferable.
Alternatively, make http:// and https:// a dropdown list immediately before the text box to allow the user to select one or other.