To overcome the below error, please follow the below steps:
This could be problem with the version of tm package installed. The above command will be work with tm package 5.1 Version.
Hence Remove the package:
Downloading the tm package 5.1 version from the below URL:
http://cran.r-project.org/bin/windows/contrib/3.0/tm_0.5-10.zip
Install the package with the below
Command: install.packages("tm_0.5-10.zip",repos=NULL)
Please run the below command to check the version of tm installed.
packageVersion("tm")
and then run the DocumentTermMatrix and check.]