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An EPUB file is essentially just a ZIP file with the file extension .epub. It is a container for the files that contain the book content and formatting ((X)HTML, CSS, images, etc), and a few other files with metadata (XML files). The "content.opf" file is one of the metadata files. The errors you're getting indicate that several of the "id" attributes (and one "idref" attribute) inside the content.opf file have values that aren't allowed in the OPF or XML specification. The numbers in brackets indicate the line number where the error is located in the file.
How to fix - that depends on whether you're creating the EPUB by hand, or if you're using an application to create the EPUB. If you make a copy of the EPUB (so you don't risk messing up the original), then change the file extension of the copy from .epub to .zip you can extract the OPF file and open it up in a plain text or HTML editor to have a look at the content and edit it. If you're not sure what to look for, post a copy of the content.opf file here, and I or someone else will have a look. If you're using an application to create the EPUB file, let us know what it is so someone can suggest the best way to fix these errors.
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