Impossible to drag and drop a module on an existing page

Hello,

I am using as hueman theme but I don’t like my article presentation because it
is limited to 2 columns (2 columns of images with text extract below),
so i want to increase the number of items per line but i can’t do it
and no options in the theme.
I just installed nimble builder, I have the bar displayed at the top with the + by clicking on it (add content), it displays me on the left side
“select a content type” with 3 buttons “Pick a section”, “Pick a module”, “Pick a template”, so I choose “pick a module” and I
drag and drop (the Post grid module) on my article area to be able to modify my existing page
but it always shows me a crossed out circle (like impossible manipulation…??!!!) and even on whatever the area of ??my page.
Did I miss any steps in using nimble. ???
Rem: Should we perhaps create sections beforehand? but my already existing page does not contain any
(I only have my articles area) if so how to create sections ???

(or is there a simpler way without using Nimble to change the number of articles per line ?)
Thanks a lot !!