There may be a maximum file size, but this is not announced in the help pages. The largest file I have ever uploaded was about 130 MB. - If you started adding the book with uploading the PDF, this may have caused the problem. Better click on "Skip", then type in the bibliographic data, and upload the PDF file only after the page will have been created. If this will no work, you may reduce the file size, e.g. with the free program PDF24 (https://www.pdf24.org/en/).