Hi Anticus, welcome to MobileRead!
This section here is for uploading books into our own MobileRead library which I think is not what you're intending.
Maybe you can repeat your question in this thread
https://www.mobileread.com/forums/showthread.php?t=54309
or in the "Apple Devices" or "ePub"-section or ask a moderator to move it to an appropriate place.
And it would be very helpful if you could tell us the error messages.