Can't import ebook

Whenever I try, there’s always an error. It says make sure you removed all digital protection. I don’t even know what that means. Is it because of the size or something? I tried different books though, and they all are between 1mb-6mb. Appreciate any help in this.

Seems like ebooks you are trying to import are under the copyright protection (DRM protected files) and can’t be imported.
Looks at this thread for some tips related to importing the protected files:

Thanks for the reply. I checked all the books, they are unencrypted PDF files. Apparently, I can text, print, copy, make changes, and so on. So, there is no protection whatsoever. I’ve tried importing multiple books and unfortunately, the result is always the same.