The SVN url has been changed by SourceForge. In your repository root, or in an individual module, run the command:
svn switch --relocate https://apertium.svn.sourceforge.net/svnroot/apertium/ https://svn.code.sf.net/p/apertium/svn/
Staging is where language pairs go that have been extensively developed, but have not yet reached release readiness. They might be missing small portions of code, or some parts might need to be tweaked. They may also have quite advanced development, but low coverage. It should take around 1—4 weeks work to bring the pair to trunk level (release readiness).
It can be found here.