Some readers had problem with the stemCompletion step in section 10.3 - stemming words. It is a bit tricky to set it up correctly.
Please check if Java is installed, as well as R packages Snowball, RWeka, rJava and RWekajars, as suggested at the beginning of section 10.3. If one or more of them are missing, you may still be able to run the stemCompletion code without error, but it would not actually do any stem completion.
If it still does not work, a suggestion to skip stem completion, if it is not absolutely necessary in your work, so that you can go ahead with text mining.