@Louise_1 is right, To create a floating or sticky sidebar placeholder you need to first add a floating Ezoic placeholder.
Take a look at this article : https://support.ezoic.com/kb/article/using-the-ezoic-chrome-extension and the one Louise shared.
Under the placeholder settings ^, where you see ""where on the page is this placeholder"", select ""sidebar floating"". Next give the placeholder a name, select devices, sizes and alignment as usual. (See article above for how to do this), then save the placeholder.