I've enabled the left sidebar but is not showed

Hi there,

by default Side bars are disabled on pages with Sections. This article explains the code you need to activate them.

https://docs.generatepress.com/article/allow-sidebars-sections/

Thanks for the very quick answer! With this hack i've solved the problem. But is weird that GP have the option to show/hide sidebars but this not works without hack.. not?

Glad to hear you got it resolved. Sections were designed to be full width, so this option was never incorporated.