Can't transfer book bought on Apple books to Adobe Digital Edition

I bought a book on Apple Books and want to read it on my e-book reader. My e-book reader is an Adlibris Letto Frontlight and requires ADE for book transfers that aren't bought directly in the company's own e-book store. For some reason, I only get error messages when I try to drag & drop and if I try to "add new file" the epub-file is greyed out, so it might have to do with copyright or something? What do I need to do in order to be able to transfer this book to ADE? 

See attached screenshots.