I'm a total nube to Flows so if my solution is not elegant, "Oh well".
I was banging my head up against the wall with this same problem.
The "sectionid" field does not work for the "Create page in a section" action, even if you pull it from a "Get Sections in Notebook" action. The flow just does not read it or recognize it.
I noticed in the description of the section id that it says you can use the id OR the url. I also noticed in another test that I did that when you manually select the section from the drop down menu, run the flow and look at the output, the flow uses a URL, not the big long section id number.
Bottom line: you can create the page if you use the PagesURL field as the sectionid.
I used "Get sections in notebook", filtered the results to the section I wanted, put the value into a string variable and then used that value in the "create page in a section" action and Voila!
Here is a screenshot. Ignore the extra steps, they were other attempts at troubleshooting the problem.
I left out identifying info. You can see part of the URL.
