Error Importing Files Into New Created Ebook

Hello,

I have a bit of a problem. I have a folder with several plain html files in it that I want to make an epub out of with calibre's ebook editor. I tried to create a new ebook. But when I tried to "import files into book", I got this error message:



Perhaps related to it, when I tried to use the "open folder (unzipped epub) as ebook" to see if it would work, it didn't either, and I got this error message:



Of course, I understand now that this second option would not work for lack of the in this message mentioned xml file, since this folder really is not actually an unzipped epub. But I don't quite understand the first error message. The html files I'm trying to import into a newly created ebook are utterly plain (it doesn't get any plainer - it has the usual...what's the phrase I'm looking for? you now, the whole html, head, title,meta etc. template, and then a body with absolutely no styling or images whatsoever), and check out just fine. In fact, each and every one of them converted into epubs via the editor's "import html or docx file as a new ebook" just fine.

What am I not understanding here? I use the calibre editor to create new ebooks all the time, and I never had such problems.

Thanks in advance for your help. :)