You'd need to say more. Generally Zotero supports adding citations in footnotes. When exactly are you getting that error message and what does it say exactly (don't translate)?
Not a general issue in Libre Office -- looks like you may have "Bookmarks" selected in the Document Preferences? As indicated there, those don't work in footnotes. That's a LO limitation, nothing Zotero can do about that.
Thanks, but since I must save and send to other people in docx format, I also must use bookmarks. Previously when I used ReferenceMarks, I have lost references and had to reinsert them... If I contact LibreOffice Writer developers, do You think this is something that can be solved/changed in the future?
When I share with collaborators I always send a copy of the document that has had Zotero fields removed (flattened). Switching between document formats and having other people editing the document itself, perhaps without sufficient care to the fields, can do permanent harm. If I accept the colleague's changes I will manually add them to my working document that has live fields. Also, I save multiple generations of my working document to allow me to go back to a version that is intact if something happens to the current version.
But obviously flattened files as a drag if you're actually collaborating. I just don't think there's a good option for collaboration (i.e., frequent exchange of documents) between Word and LibreOffice. There's Google Docs as an alternative, there's the option to work without footnotes, until the final stages, there's the general transfer between all supported word processors (https://www.zotero.org/support/kb/moving_documents_between_word_processors#word_and_libreoffice) but none of these are great. I don't think you'll have much of a chance of getting anywhere with this at LO, to be honest. THey already support Reference Marks, which work very well. I'm not even sure if the problem is LibreOffice or the way the "Bookmark" property is defined in .odt and .docx.
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If I contact LibreOffice Writer developers, do You think this is something that can be solved/changed in the future?
I don't think you'll have much of a chance of getting anywhere with this at LO, to be honest. THey already support Reference Marks, which work very well. I'm not even sure if the problem is LibreOffice or the way the "Bookmark" property is defined in .odt and .docx.