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/
The incubator can be found here. It provides a place for people to put dictionaries and other stuff that is useful in constructing language pairs.
Basically if you've done some work on a dictionary, but it doesn't fit into a language pair yet, or you just want somewhere to put your work so others can improve it, this is the place. It means that work doesn't get lost on people's computers, unavailable if someone else wants to work on it.