X_ASSET_MISSING
Means
a page names a site asset the app does not have
Typical cause
asset('assets/…') names a file that is not under apps/<app>/site/assets/, or a path outside that tree (.., an unsupported extension). Thrown while the page renders, so x build --target static fails on it
Fix
add the file under apps/web/site/assets/, or correct the path passed to asset(); then x build --target static. Details: Static Assets