PowerPoint files can only be used to create Claro content. This is because PowerPoint pages have fixed dimensions plus content elements on the page are added in layers, which aligns with how pages are structured in Claro. Flow pages are structured very differently so they can resize based on the screen they are being viewed on, and that difference means PowerPoint files can't be easily converted into Flow content.
If your PowerPoint file is more than 20MB, divide it into two or more files and import them separately. You can then merge the imported content back into one course using dominKnow | ONE's Copy From feature, which lets you copy content from one course or project into another.