TextX is a meta-language (i.e. a language for language definition) for domain-specific language (DSL) specification in Python.
textx-mode is available on MELPA and MELPA stable.
You should enable MELPA in your emacs.d as suggested in documentation.
To add MELPA stable follow the documentation from here.
Then you can install textx-mode with M-x package-install [RET] textx-mode [RET].
If you have enabled both MELPA and MELPA stable you can pin textx-mode to be fetched from MELPA stable:
(setq package-pinned-packages '((textx-mode . "melpa-stable")))