not able to set another parent page except main page

Hello.
I am not able to set any page as a parent page.
WP wants all section pages be under main page, like
domain.com/main-page/products/
domain.com/main-page/about/
domain.com/main-page/products/food/ and so on
In other case if i try to set domain.com/products/ as main page (no parent) then i get a mistake “It seems we can’t find what you’re looking for. Perhaps searching can help.”
In case if i set 301 redirect from domain.com/main-page/products/ to domain.com/products/ i again get the same problem – “It seems we can’t find what you’re looking for. Perhaps searching can help.”
What can be a problem?
Thank you.