This error is not due to the non-input of web site.
See here: [Does not allow me to create a new app][1]
[1]: Does not allow me to create a new app
In addition, if there is no one that still write in “Website”, you can insert a placeholder.
(If you don’t have a URL yet, just put a placeholder here but remember to change it later.)
It is written in fine print at the bottom of the input field of the “Website”.