Nearpod does not support the Keynote file type. The good news is you can convert the file within Keynote.
1. Open Keynote and select the document or, as shown, choose to create a new document.
2. Select a theme
3. Once you've finished creating your presentation, click File > Export to > PDF or PowerPoint (the two supported files you can export to)
4. Choose the options you wish to save as and then select next. You'll be prompted to confirm after.
Now, you're ready to upload your lesson into Nearpod. To learn how, read this article.