Why do I see code beginning with ADDIN ZOTERO_ITEM CSL_CITATION in my document instead of formatted citations?
By default, Zotero stores the reference data for citations and the bibliography in Fields (Word) or Reference Marks (LibreOffice), which stores items' reference data hidden behind the formatted text.
If Word or LibreOffice is showing the field codes, rather than the formatted text, you can hide the field codes by pressing Alt/Option-F9 (or Option-Fn-F9 on a Mac) in Word or Ctrl-F9 in LibreOffice. For a single citation in Word, you can right-click on the field code and choose Toggle Field Codes.
If you see field codes showing repeatedly, check your word processor settings for displaying fields. In Word for Windows, open Word Options, then choose “Advanced” and uncheck the “Show field codes instead of their values” box. In Word for Mac, open Word → Preferences → View and uncheck the “Field codes instead of values” box in the “Show in Document” area. In LibreOffice, open Tools → Options → LibreOffice Writer → View, and uncheck the “Field codes” box in the “Display” area.
When using Track Changes, Word or LibreOffice will sometimes show you the changed and original versions of changed field codes. If this occurs, click “Accept” twice to quickly accept the changed citations and hide the field codes again.