It would appear that your books are not DRM free...and it might be possible that they are not epub. I'd try downloading Calibre (it's free) and import the books and export to epub. If they aren't DRM'd, this would likely work for you.
If they are DRM'd -- you'll have to search for a way to relieve them of that burden
Or -- you could always just search for a copy of the book on the 'darknet'. I never feel bad using the darknet to get a book I've purchased in a form that's free of drm.