Hi Akki ,

First you must make sure the page is a Blog Template and not a Layers  Page .
You can go to dashboard-> pages-> name of page -> then under template select blog then save.

the if you have made a post and want to add widgets to the sidebar you go to the post doesn’t matter which one .
then on top click customize then in the customizer go to Edit Layout -> Right / or left sidebar -> add widget.

this is for single blog posts , you can do the same with woocommerce product page / Product list page with storekit and also with the blog post list.
you just have to go the the page you want the sidebar to be on and then click the customize on top.

Dieter