This appears when a file has no text inside for Studio to read. Scanned PDFs are pictures of pages, so they show this message.
Other file messages:
"Could not read that file. Try another format, or paste the text instead."
"Use a PDF, Word (.docx), HTML, Markdown or text file."
"Files can be up to 10 MB."
What to do
Export the document from its original app as PDF or Word (.docx), so it keeps real text.
Or copy the text and use Text in Create.
Split very large documents and import the part you need.