I am trying to add an iframe that pulls from a live Google Sheet. I know I have to set this up as a mediawiki page and add it as a Verbatim tag (at least that's what a search is telling me) but it tells me I don't have permission to edit MediaWiki pages. I'm the wiki owner.
First off, there is no such thing as "wiki owner". You can be the bureaucrat or an admin, but Fandom owns wikis, not the users.
Also, if you want to add an iframe using a Google Sheet, you do not have to set this up as a MediaWiki page, but you can use the extension GoogleDocs4MW. You do not have to install it, and you can use the instructions as mentioned as use there. Fandom uses a slightly different version of that, but the code on Github doesn't have any instructions, so it might be easier to follow them on MediaWiki.
Essentially, someone used HTML to create an overlay of the log in area, and suckered several people into lohhinh in to that. Including one staffer. With the credentials of this staff account, the vandal then deleted a couple of big wikis and redirected others to My Little Pony and such.