New document - Left master page missing

Hello, I created a new document in Framemaker 2020 (file>new>document>portrait) (my old Frame 8 templates are not working despite the settings appearing to be correct - the text will not flow onto multiple pages). I only get a Right master page. I try to create a Left master page but the dialogue says this label is reserved... which makes sense... but how can I edit and use the Left master page!?