EPUB refusé par une librairie en ligne

I have no idea why an EPUB export would be seeking an 'external text anchor.' Can you identify the document element that this error is pointing to, and perhaps change the way it is linked into the document?

Also, more of an opinion than an absolutely fact, but most of those e-book sellers are of such minor importance that I'm not sure they are worth considering for either error correction guidelines or uploading. Except for some niche works and genres, about 95% of all e-books are sold through Kindle (by far the largest market), with some through Apple and Google. The rest, even SmashWords, are... trivial. (I don't think I've ever sold a title, across a range of categories and types, in any of those minor marketplaces.) I just would not worry about anything but Kindle (conversion) and Apple (optimized EPUB) unless you have good evidence your work would sell on those minor outlets.

You also need to be aware that there is next to no such thing as an EPUB standard out in the actual marketplace. Every EPUB reader has its own quirks and demands, and a book that is optimized for one may have all kinds of rendering and readability faults on another. It is common for a "perfect," plain-vanilla EPUB that passes validation and reads on the simple readers like Calibre and EPUBreader to fail Apple's standards, or have display problems there that need to be addressed... making the doc faulty on other readers.

So spending time getting a version that will pass each seller's standards (or, to be fair, each group of sellers with similar standards) may be a lot of fussy work that ultimately has no return value.

But as for that error, it's a strange one and I can't even guess what's causing it unless you can identify that object or a link within the book that is trying to connect to an "external" element.