If you have made your own ePub file, or have generated one through a different conversion service, make sure that you run the file through an ePub validator before uploading it to your Kobo Writing Life account. Note that there are many free ePub validators online.
ePub files will fail to upload to your Kobo Writing Life account if the file size is too large or if there are errors within the file.
If errors are flagged when you run your file through an ePub validator, be sure the correct those errors and run the ePub through the validator again before uploading the file to your account.
Files with errors in them may display properly on our devices and in our applications, but in order to guarantee that your readers can view the file on any device, we do not allow for files with any formatting issues. You will therefore receive an error message at file upload if your file contains any errors.