X_ASSET_VARIANT_UNDECLARED
Means
an /assets image variant responsiveImage() never mints
Typical cause
?w=, ?f= or ?q= on a hashed /assets URL names a width, format or quality outside the declared set for that source (usableWidths over DEFAULT_WIDTHS, DEFAULT_FORMATS, no quality), or the file is not a raster the pipeline decodes
Fix
responsiveImage({ src: asset('assets/hero.png'), width, height, alt }) — no widths/formats override for a site asset; an upload under /media answers any width