I am not able to create article.

Hello. How to add regular pages under Site Structure?
It seems like it is necessary to have a regular page under site structure in order to create an article. When I try to add regular pages there, it gives the error display "Top-level pages must be website root pages!".
Any selection other than website root pages gives this error there.

Any suggestion would be appreciated. Thanks in advance.

Every site structure must have at least one website root page! Only then you can create regular pages as subpages of the website root. In the Contao Online Demo you can see how a page structure should look like: https://demo.contao.org/contao/login

Thanks a lot for your help.