If you plan to let the tool generate individual language models per text, you have to get (essentially build) KenLM. Before doing this, you should install its dependencies. For Debian based systems this can be done through:
$ sudo apt-get install build-essential libboost-all-dev cmake zlib1g-dev libbz2-dev liblzma-dev
With all requirements fulfilled, there is a script for building and installing KenLM and the required DeepSpeech tools in the right location:
$ bin/lm-dependencies.sh
If all went well, the alignment tool will find and use it to automatically create individual language models for each document.