What is a Site Preferences Stack?

Hello. I’m receiving a message on occasion (for example when I want to use a table… or when I try to embed a PDF) as I attempt to build with Stacks in Rapidweaver 8 which says:
“Please add the Site Preferences Stack to the page, otherwise the Platform will not work correctly”.
I have no idea where to find this “Site preferences Stack” and have exhausted all the options I could think of… so am writing to ask can anybody help point me in the right direction to find this? Thank-you, Stephen

Are you using the Platform framework or trying to incorporate stacks from it?